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By Pete Snidal © 2001, Revised July 2007

This little text file began as an email I wrote outlining the most basic “need-to-knows” about air brakes. it is not intended to be a substitute for proper air brake training and certification, but I suggest you read it carefully, if for no other reason than to learn what you don’t know – and why you need to know! First, let me say that in all of Canada, and I’m sure in most if not all of the States, you must have an air endorsement on your driver’s licence to be permitted to drive an AB equipped vehicle. Although I’m hardly a fan of regulations, I almost have to agree with this one. It’d be a very irresponsible thing to do to drive one without having taken the time to master the details of Air Brakes. Following is a brief discussion of the differences between the two types of systems. This is not intended to make you an expert, but rather to motivate the reader to seek the education – and certification – to run an airbrake-equipped vehicle intelligently – and most important – safely.


Failure to recognize – and compensate for – the difference in Air Brakes will inevitably result in mayhem – and probably death. Although there have been many regulations imposed in the past 40 or 50 years in various jurisdictions, it still happens all the time, although fortunately less often.

A Scary Scenario..

Imagine this. After lots of searching, you finally found that wonderful classic bus conversion you’ve wanted for years. And now you’re driving along merrily in your “new” 20,000 lb.motorcoach. It runs wonderfully, it steers great, and it stops on a dime! Those Air Brakes are just wonderful! You haven’t done too many miles, yet, but every time you’ve stepped on that big air brake pedal, you’ve been really gratified at how well it worked! So, now, as you approach the top of this downhill grade, you do so with great confidence. After all, those brakes have worked GREAT so far – what could change? The first time you use the brake, they work like they always have – so far. The second time you apply them, same thing – slows it down from 50mph to 35 in a heartbeat. But the next time you use them, NOTHING happens! Nothing! The thing just rolls blithely on, as if you weren’t using the brakes at all! What could be wrong? You check the pressure gauge on the dash – it’s well in the green, lots of air pressure. You’ve been keeping an eye on it; after all, everybody knows air brakes need air. So you hit the pedal again. And again! (What else can you do?) It feels just like it always has, and you hear the air whoosh back when you release it, but when it’s down, there are NO brakes! Finally, in desperation, you hit the parking brake release on the dash – the so-called “maxi” brake. Still nothing! You’re in trouble! This kind of trouble:

This coach used its last brake application before the attempted one that had this result. There was NO difference in pedal feel between the first one after adjustment and the one during which this happened! Fortunately – and miraculously – there were no serious injuries but you can bet the driver has become a Christian about brake adjustment!

What went wrong? Can this really happen? Yes it can, and it WILL – unless you began your bus adventure properly equipped with the knowledge that air brakes are different! They must be checked and adjusted as the shoes wear, and this must be done on a regular and frequent basis. Meaning daily!

May I Emphasize This? It is impossible to overemphasize this. Let me say it again: Airbrakes are different! They must be checked daily!

Now let’s look at why:


1. The Differences

If you are to operate an airbrake-equipped vehicle, you mustunderstand the differences between this type of brakes and those of the vehicles with which you are already familiar. The first and most major difference is the complete lack of “feel” in the brake pedal, a feature to/with which we all become accustomed/complacent – with the hydraulic brakes found on virtually all passenger cars and light trucks.

2. How Air Brakes Operate

The first need-to-know before even starting an airbrake-equipped vehicle is that air pressure is necessary for the brakes to function! There have been occasions on which an inexperienced driver has started an older air-brake vehicle, such as our Flx Clippers, released the mechanical parking brake, put it in gear, and drove off, before the compressor has built up sufficient air for the brakes! If s/he has to stop before the pressure has had time to build up, application of the brake pedal will of course result in NO BRAKES! Even at parking-lot speeds, this can end up doing a whole lot of damage! Although this is not possible with the later “spring brakes,” or “maxis,” (see below), it can be done with the old mechanical parking brake. And in this case, until the air builds up sufficiently, this will be your only brake! For this reason, they are equipped with a low-pressure warning light and buzzer, but these may not always be functioning. The driver has to know to look for this, and mostly to ensure that adequate air pressure has been attained before releasing the parking brake.Common sense, but not a requirement for the hydraulic-braked vehicles with which the new air-brake driver will be much more familiar. I mention this only to offer a “heads-up” to the absolute newbie to air brakes, just in case. In the more common hydraulic system, the power which applies the brakes comes from your leg; with air brakes, it comes from the compressed air tank, your leg only opens the valve to let it out. No airee, no brakee! The stock Flx air pressure gauge (the most important instrument on your dashboard!) is marked with a “green zone,” and your indicator must be in this zone before you release the parking brake. Minimum pressure is in the 80 psi range.

Does it “Kick Off?”

While the experienced driver is warming up the engine, and watching the air buildup, s/he also ensures that the air pressure ceases to build at the other extreme of the pressure range. The Air Pressure Regulator – part of the compressor – must release at the top end of the gauge – 120 psi. This is the point at which the gauge stops climbing, and that a difference may even be heard in the sound of the compressor, if the engine is otherwise quiet enough. If it doesn’t stop climbing, s/he shuts the engine off immediately, since the compressor has the power to do some serious damage if the last resort – the safety, or blow-off valve – fails to step in and deal with the problem. Since these get so very little exercise in normal practice, it’s best not to count on them.

Does It Recover?

The experienced operator, at the beginning of each day’s use of an AB vehicle, will then do a repeat test of the air system, by “fanning” the brake pedal until the pressure drops below the cut-in point, and check to see that the compressor once again cuts in and builds up pressure in the reservoir tanks. He will then shut the engine down, and do a walkaround, listening for any air leaks that may be present, correcting them or calling in professional help to remedy the situation. He will also listen for leaks when the brakes are applied – this is best done with two people, a listener and an applier.

All-Important: Adjustment

Only once he’s satisfied that the system is secure will the experienced drive proceed then to the final check – that of the all-important adjustment– the individual linkage between the brake air chamber and the actual brake shoes at each wheel must be within tightly-defined limits.

Constant Monitoring While Driving

Needless to say, during operation of the vehicle, the driver must always monitor the operation of the air pressure system – the compressor must cut in and build pressure at the lower level of the green zone, and cut out at the upper level as the vehicle is driven about the countryside.

2. Operation Of The Brakes Themselves

The “business end” of the air brake system. Air is admitted to the air chamber through the treadle valve, or brake pedal. Air pressure behind the diaphragm in the air chamber forces the push rod to move the end of the slack adjuster – the adjustable lever which rotates the S-cam shaft. The rotating S-cam forces the brake shoes against the drums. The resulting friction dissipates mechanical energy into heat, thus slowing the vehicle down, and also wears down brake lining during each application. Thus, every time the brakes are applied, more movement of the air chamber pushrod is required than for the time before. The resultant increase in slack in the system must be adjusted out regularly – well before the limit is reached!

The Pedal: A Valve, Not A Pump

Of major importance is the difference between air and the “normal” hydraulic brakes with which all are familiar – the air brake pedal is not a Pump, as with hydraulic brakes, but rather a Valve. Operation of the brake pedal – correctly called the Treadle Valve – admits air from the reservoir to the brake (air)chambers to apply the brakes – providing, of course, that there is enough air in the reservoir to do the job – and also, that the linkage is sufficiently tight between air chamber and brake shoe. The linkage between the air chamber and the brake shoes is external, to permit regular and frequent checking and adjustment.

With hydraulic brakes – the type we’re all used to in our cars, as the linings wear, the “feel” of the pedal changes – it takes more pedal movement to take up the slack as the shoes move farther before contacting the drums. This continues until the brakes are adjusted – the physical linkage between the wheel cylinders and their brake shoes is lengthened by a screw adjustment. This is done by your mechanic during brake service, or to a certain extent by “automatic adjusters” in the wheels, which tighten up the linkage each time the brakes are applied in reverse. But – and this is important – even if brake adjustment is neglected, hydraulic brakes can be “pumped up – the pedal can go all the way to the floor, but a second application will just send more fluid to the brake cylinders, and braking will still occur! (Providing, of course, your brake fluid reservoir level has been maintained!) This is because, with hydraulic brakes, the master cylinder – operated by the brake ]pedal – is simply a pump– each stroke adds more fluid to the system until all the slack is taken up, and the shoes contact the drums.

***Update: Disc Brakes The foregoing discussion of hydraulic brakes applies to drum brakes only – the later disc brakes require addition of fluid as the disc pads wear. Since this article is about air brakes, we’ll go no further. Suffice it to say that the “pumping” action of hydraulic brakes is less pronounced with discs. ***

But, getting back to Air Brakes, no such “pumping” is possible! When the adjustment is gone, it’s gone! – the pedal is not a pump! It is simply a valve which directs air flow to the brake chambers – these contain diaphragms by which air pressure is translated into force on the brake shoes against the drum. These Air Chambers have limited movement. As the brake shoes wear, more movement is required on the part of the push rod to apply the brake. The brake linkage must be tight enough so that the brakes are applied before the limit of travel of the air chamber is reached. Once the maximum movement of the air chamber is reached, no amount of “pumping” will have any effect on the non-action of the brakes.

NO Difference In “Feel”

Once the chambers have traveled their full extent, if the shoes are still too far from the drums, there’s NOTHING that can be done from the cockpit! And there’s no difference in “feel,” – whether the brake chambers need only a little bit of air – tight brakes, or almost all they can stand – loose brakes. The treadle valve feels the same – just as opening a tap to fill a bathtub feels no different from opening one to fill a washbasin!

An analogy as it was once explained to me is that of the handlebar brake lever on a bicycle or motorcycle. If the cable is too loose, the lever will contact the handlebar before the brake is applied. No amount of extra squeezing can help at this point, the cable must simply never be allowed to get this loose. But the difference is as the slack increases, you can see it! You can’t see the slack building up in your brake linkages unless you get down under there and check it!

There is no other way. This is the reason for the signs on the mountain passes saying “Truckers – Stop Here – Check Brakes.” At these spots – as well as at many others, truckers – and other operators of airbraked vehicles – must “get out and get under,” to check and/or adjust brake slack. That failure to do this can have disastrous consequences is dramatized by the photo above.

One At A Time

There is an insidious aspect to this “creeping brake failure” as well – the brakes seldom all fail at once; rather they go one at a time. If you’re using just a little braking, you probably won’t notice that you’re down to 3 brakes, then two, then, possibly, even one! But this reduced braking is the only warning you might get that your braking is eroding away as you drive!As loose brakes wear just a little bit more, you get a situation in which the chamber moves its maximum, but the brake on that wheel still isn’t engaged! This “feels” just the same to the driver – depressing the pedal allows airflow to the brake chambers, but when the chamber is full and stops moving, the brake for that wheel is not applied – but in the light applications you normally use, you have no way of knowing that you’re now down a brake – or two!

A Wear Spiral

And of course, the remaining brakes which are still working are taking more load, and wearing faster than previously, thus running out of _their_ adjustment! And of course, your stopping distance in a panic situation has dropped by the power of one brake. Worse yet this situation will continue for the next brake to run out of adjustment as well, and now you’re _two_ brakes short! – Until, finally, if you don’t catch it, you’ll be down to NO brakes! – And this entire scenario can easily play out on one good-sized hill!!

And remember – when this happens with ABs, all the “pumping” in the world will do you no good; the chambers are at their full extension, and the brakes are too loose to care. This is why you see those “Trucks Stop – Check Brakes” signs at the tops of the big hills on the highways. This is also the reason for the laws, which don’t seem to have stopped the articles in the news we still see regularly about trucks “losing their brakes” on hills once in a while and wiping out carloads of civilians. Happens here in BC a few times a year, even though the drivers have passed the air brake test, which has come to be another government formality which too many drivers apparently feel just needs to be gotten around, but to which they otherwise pay little attention. (You can lead a horse to water, but you can’t make him think!)

What To Do?

Avoiding this ugly scenario is clearly the responsibility of everyone who drives any airbrake vehicle – even across town or around the block. You must at all times know for certainwhat the adjustment status of your brakes is – as well as the state of your air supply.

Fortunately, the latter is simple enough to check – you will have a reservoir gauge on your instrument panel. This gauge tells you the pressure of your reservoir – it will fall just a bit each time you apply the brakes, and rise as the compressor “kicks in” and repressurizes to its pre-set limit. More on this later.

The adjustment of the brakes themselves can be checked in only one way – getting out and getting under, and physically checking the slack in the linkage between air chamber and brake linkage.


In order to check the brakes, they must be off – ie, no pressure to the brake chambers, the diaphragms being fully back. This means the vehicle can move, possibly rolling over the brake checker, unless some other means of limiting movement is used. Thus, even on level ground, only the very foolish will fail to carry a pair of wheel chocks, which will be in place before any rolling under the wheels is done. If you have a reliable mechanical driveshaft brake, or an automatic with Park position, or standard gearbox in gear with engine off, you may choose to rely on these alone, but the best approach is the ol’ “belt-and suspenders” one – be absolutely sure you’re not going to get any surprises while you’re laying under your tires! And of course, you can’t leave an air maxi-brake (spring brake) on and adjust the rears – more on this later.

Adjusting out the slack

Tightening the worm adjusting nut rotates the clevis about the cam shaft, taking out the slack. Note the two adjustments – slack adjuster position and pushrod length.

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The Single-Pot Brake In The Off Position. Note that both the shaft clevis on the shaft and the pushrod length are adjustable.

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The On Position. Note that the brake clevis is  perpendicular to the push rod when the slack  is taken up. Note also that the pot is at the limit of its travel. – Past Adjustment Time!

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As shown by the drawings above, (cadged from the BC Air Brake Manual – with value added) the diaphragm in the brake chamber has only a certain amount of movement. There must be enough slack in the linkage to allow the brake shoes not to contact the drums when the pressure is off, but there must never be too much slack, which will allow the diaphragm to reach the limit of its travel before contact is made. Note also that the ideal angle between the pushrod and the brake shaft clevis is 90 degrees at the point of shoe-drum contact. This gives the greatest mechanical advantage. Periodic Pushrod length adjustment is necessary to maintain this situation, but the necessity crops up much less often than the daily (or less!) requirement for slack adjustment.

Diaphragm distension is not limited by applied pressure – the movement limit is reached with only a few psi; after that, increasing pressure (by pedal movement) increases brake action.

Checking Adjustment

Once out and under, checking the actual adjustment of each brake pot – there is one behind each wheel – is done by physically manipulating the brake clevis from the no-pressure position to the limit of full movement. This is most easily accomplished with a lever-type tool – also an adjustment wrench, called a “Brake Buddy” (TM). But you can do it in a number of other ways – the strong will do it with their bare hands. Important thing is to ensure that you check the full movement! If the pushrod travel exceeds 1/4-3/8″, adjustment should be done. If it’s over 1 1/2″, you’re lucky you’re still alive! it’s time to tighten the worm nut and re-check, repeating the process until the adjustment is correct.

Another Possible Problem

Some drivers don’t pull on the end of the slack adjuster to check slack. They’ve gotten lazy, and they just use their wrench to tighten the slack up completely, then back off a half turn or so. But there is a serious danger in this method! It’s possible to be loosening the linkage all the way, and then just tightening it that half-turn! – Which is of course an invitation to disaster! Bear in mind also, that “lefty-loosey” and “rightey-tightey” don’t always apply with slack adjusters – they can be installed upside-down, so this rule won’t apply. For this reason, you should always pull the slack adjuster away from the air chamber, and see how far it moves. More than 3/4″ means it’s time to tighten it up till there’s no movement, then back it off a half turn or so for about 1/4″ of movement when you – and this is important – re-check.

How Often?

Legally, all trucks must stop at every trucks-stop-check brakes area and check slack – and you should, too!. While walking around, you should also listen for any hissing air – sign of a leak developing or developed. It may be argued that Motor Homes and Converted Busses, being lighter, stress their brakes less and can handle more extended periods between checks. What is important and indisputable, however, is that you must check often enough to be sure that none of your brakes is ever too slack! Check them often, until you get a “feel” for how much wear you’re doing on the road. Obviously, cruising the prairie, you’ll likely find you can go for days without finding much wear/need for adjustment, whereas a few hours in the mountains can wear them down enough to need it. Use your head – and when in doubt, check!

“Two-Man” Adjustment Check

There is an easier way to check your adjustment – if you have a co-driver, that is. That way is just to stop, chock the wheels or otherwise prevent movement, and have the co-driver give a light application of the brakes each time you position yourself to observe the movement of the brake mechanism for each wheel. If the movement exceeds the limit, then you have to get under there and tighten/re-check, but if you’re within limits, you can just carry on until the next check. There are two advantages to this method: the first is, it’s easier, but don’t miss the second: this ease should motivate you to stop and check them more often – each time you stop for fuel or a snack, for example. It takes little time and gives you a whole lot of peace of mind! Another tip: carrying a broken-down cardboard box provides a convenient “creeper” for brake checks – I keep one in my baggage compartment with my brake adjusting tool. Just throw it down under the bus, and you have a nice clean surface on which to slide under the rig. When it gets too dirty, a fresh creeper can be had at any grocery store!

Other Considerations

Now that you’re aware of the most important aspect of air brake systems, there are other considerations as well:

Pushrod Adjustment

Pushrod adjustment is required only when the final angle between clevis and pushrod at the point of brake contact gets too acute. Fortunately, this doesn’t happen nearly as often as excess slack in the mechanism. Adjustment of this parameter is usually taken care of in the shop. But you’ll want always to make observation of the angle at brake contact a part of your checks. When the angle gets too far from 90 degrees at the point of brakeshoe contact, pushrod adjustment is required. So it’s a good idea to monitor this one during your daily brake checks.

Spring Brakes – Parking Brakes for Air-Equipped Vehicles

Another attribute of air brake systems is that they can incorporate “Spring Brakes.” These are secondary spring-operated brake applicators, usually only on the rear axle(s), which apply the rear brakes for parking or emergency purposes. In the event of air pressure failure, for example, they can be designed to apply automatically. This can result in driver embarrassment – if you’re not ready for it, sudden brake application at highway speeds while you’re still in the middle of the driving lane can be stressful. But less so than finding your brakes not working the next time you need them.

Important Note

It is VERY important to note that spring brakes offer an “emergency application” aspect ONLY in cases of air system failure! They offer NO compensation for faulty adjustment. If the adjustment is too slack, once again, the springs only serve to apply maximum movement to the air chamber/pushrod, and once this limit is reached, no braking will be done regardless of pedal application or air supply failure!


Parking Brakes

The primary function of spring brakes is as parking brakes. The full force of the rear brakes is applied whenever the vehicle is parked. Being spring-operated, they don’t depend on air supply, and so stay on long after the air reservoir has lost its pressure. In this case, they are applied by means of a driver-operated valve at the driving position, which releases the air pressure from the caging chambers. They provide a further safety feature in that the vehicle can’t be moved until the air pressure builds up enough to de-apply the springs – and therefore enough to use the brakes to stop once you’ve begun moving!

Thus, the spring brakes (sometimes referred to as “Maxis,” or maxi-brakes) are always on, unless the spring is “caged” by the second additional air chamber, “piggy-backed” onto the one already there for the regular service brakes. To de-apply the parking brakes, the system must first be brought up to pressure (the compressor) and then the spring chambers are charged with air by means of the dash-mounted parking brake valve. This offsets the springs and lets the vehicle move. (In most cases, there is also a mechanical means to cage the springs in case of emergencies – you get out and under, remove the dust cap, and screw the cage bolts up by hand.)

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No-Pressure (Park) position – spring is “uncaged,” over-riding regular brake chamber and applying full brake

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Normal Driving Position – spring is caged by pressure to spring chamber, service brake chamber is unpressurized – no braking action

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Service Brake on during regular driving – Spring brake is caged, but application pressure to service chamber applies brake (Danger Note! – Spring brake chambers, obviously, have big, heavy springs in them. Attempting to dismantle them without proper information and precautions will earn you a Darwin Award!

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This kind of parking brake has a major difference over the brakes we’ re all used to – if the system loses its pressure, the de-application pressure is also lost, and the parking brakes will come on as the pressure decreases – providing an “automatic” setting of the parking brakes if system pressure is lost on the road. This is another reason we must pay attention to system pressure – if it drops to the danger level, you need to pull over in the first safe place, while you still have enough air to maintain control over your brakes, or you just may find yourself stopped in the middle of the freeway!

But Not Always…

That is, provided There is an alternative to providing the deapplication pressure from the reservoir of the regular system. In some cases, the de-application pressure is provided from a second special reservoir, supplied from the first one through a one-way check valve, so that pressure loss in the main system will not affect the pressure in the spring brake reservoir. In this case, the spring brakes will only be applied when the dash- mounted valve is operated to exhaust the air from the spring brake chambers. This eliminates the “automatic” aspect of spring brake application in the case of loss of pressure in the service brake system, but allows greater operator control in that the second reservoir can allow the operator to de-apply the brakes for such things as small movements once the spring brakes have brought the vehicle to a stop.

What About The Flx Clippers?

By the late ’50’s the last years of the Flx Clippers/Visicoaches, spring brakes still weren’t standard equipment – a driveshaft drum or disk brake was the parking brake. Many owners have upgraded their rear brake chambers to spring brakes, installing the necessary lines and dashboard valve, thus providing an extra margin of safety in the form of an excellent extra brake for emergency situations involving loss of air, as well as a better and easier-to-apply parking brake.

Your Flx may already have been converted. If not, you’d be well advised to have it done. If in doubt, when you check your slack on the rear brakes, have a look for double air chambers. If they’re there, then you’ll have to find the application valve on your dash somewhere to release the brakes. And always remember – the springs must be caged for the check of adjuster slack to have any meaning. Obviously, if the springs are deployed, there will be no slack – although this doesn’t necessarily mean that the brakes are applied!

More On The Air Supply

Obviously, you need at all times to have enough air on hand to make your brakes work. This is stored in the brake reservoirs(two tanks, and, as explained earlier, is “topped up” as required by the compressor, which cycles on and off between two pressure limits. The compressor “kicks out” at the high limit, and “kicks in” at the lower one.

Compressor volume is important. As a compressor gets old, its ability to make air fast enough deteriorates. It HAS to make air faster than you can be using it, in every situation, long hills or whatever. The way you get a handle on this one is by its build-up time when you first start the engine with empty air tanks. The compressor needs to get your air pressure into the green (over 80 psi, in the case of the Clippers) within 3 minutes, by law, (at least in BC). Mine does it in 90 seconds at fast idle.

Governor Cut-Out

The air compressor must disconnect (it has a governor arrangement) at the top pressure – 120 psi in the Flx Clippers. You can hear it with a gas engine, with a Diesel you probably won’t be able to, but the gauge should stop climbing close to the 120 mark. If the governor fails to disconnect the air compression, a “blow-off” valve on the first reservoir tank is supposed to release. I haven’t even been able to locate mine to check it, but I keep an eye on my air gauge to ensure that cut-out happens when it’s supposed to!

Governor Cut-In

The gauge needs to start climbing again (the compressor governor cutting back in) when it drops down to the low limit – around 80 psi in the case of the Flx. The BC air brake manual calls for “fanning” the brakes to release enough air to get the pressure down, and then watching for the governor to kick back in (motor running, of course) when you do your “Pre-Trip Inspection” every morning before starting out. I haven’t met too many truckers who do this every morning, but it’s not a bad idea, in the case of a rig you don’t use every day, such as an RV. And of course, on the road, after you’ve used your brakes enough to get the pressure down to the 80 psi range, you certainly want to see that needle start to climb again as the governor cuts back in. If it doesn’t, you’ll need to use what little air you have left by the time you notice to STOP IMMEDIATELY and find out why the air isn’t building back up. Your perception will be aided by:

The Low Air Indicators

You need to be advised when your air pressure drops below a safe level. The means of advising you are:

  • Your Dashboard Air Pressure Gauge – only a fool would leave home without one!
  • Your Low Air Pressure Warning Light – this dashboard light should shine when your ignition or master switch is on and your pressure is low.
  • Your Low Air Pressure Warning Buzzer – this should be sounding when conditions are as described above
  • Wig-Wags – there may be a “wig-wag” – a semaphore-like device which drops into the driver’s view from the header above the windshield when the pressure drops to emergent levels.

The Low Air Warning Cutout Button- There should be a button on the side of your cab that can be pushed to make the buzzer stop while pressure is building up on startup. This is a Good Thing designed to make you less crazy during warm-up while you’re waiting for your air pressure to build.


Checking The System

A serious benefit RV’ers have over truckers is that we usually have a co-driver of sorts, who can be a help in checking our brakes. For instance, someone at the wheel can apply the brakes while we visually check brake rod/slack adjuster movement – and while the brakes are on, we can listen for any sounds of air leakage on the application side, such as from flex hoses or air chambers themselves. Truckers, operating alone, don’t have these options, and must always check slack by manually moving the rod/adjuster – a tool called a “brake buddy,” available at any truck supply store, is invaluable here. Either way, checking actual movement is all-important (more on this below.) And they can only check for application air leaks by listening from the cab as the service brakes are applied. An application of brakes, once the pressure is up, shouldn’t cause the gauge to drop more than 5 psi. There should be no hissing of air with the brakes applied, only a short release when they’re released.


If your rig has spring brakes, you must not apply the service brakes while the spring brakes are engaged (de-pressurized.) The pressure of the springs, added to the pressure of the service brakes when applied can cause serious problems – there’s a lot of force there!

Furthermore, it is of course useless to check slack of the rear brakes if the spring brakes are applied, since all slack will be taken up. Slack must be checked on level ground, and/or with the vehicle prevented from rolling with chocks or a driveshaft parking brake, and the spring brakes caged (pressurized.)

Any Leaks?

Once your air gets up to pressure, shut off the engine, and walk around the bus listening for any leakage of air. You will of course need to correct any you find. Then have someone apply the brakes as you check the movement behind the wheels one at a time. If slack is over 1/2″, adjust the slack adjustment levers – not the pushrods. The SALs have a bolt at their base which causes an internal worm gear to rotate around the S-Cam shaft to tighten things up. They should be tightened until it’s no longer possible to move the rod, then backed off 1/2 to 3/4 of a turn of the adjustment bolt.

SERIOUS WARNING: Drivers have been know to check slack not by attempting to move the rod or slack adjustment lever, nor by observing movement when someone else applies the brake, but just by “tightening” the adjustment until it no longer moves, then “backing off” a half-turn or so. Trouble is, you could be going to full loose, and then, when movement stops, and you think you’re “backing off, you’re actually moving towards slightly tight. “Lefty-Loosie/Rightie-Tightie” doesn’t always apply. YOU MUST CHECK MOVEMENT WHEN ADJUSTING!


The Reservoir

With the Flx Clippers, you’ll find your two air tanks just ahead of the left rear wheel. There are drain cocks on the bottom – to drain out any water or oil (usually both) lying in their bilges. Momentary opening of these cocks is not enough – it’s best to pop the cocks at the end of a day’s run and let all the air go out, taking the sludge with it. Water accumulates from condensation, and some oil will escape from the compressor – too much oil is an indication that it’s time for attention to your compressor, such as new piston rings.

Spare Belt!

Needless to say, if your compressor drive belt (if fitted – some diesel compressors are gear-driven) goes south, your safe driving time is measurable in minutes. You will notice, if you’re observant, that the gauge goes down past the “kick-on” pressure and keeps dropping. If you’ve failed to notice that, there will be other indications (see below) which hopefully will give you enough time to find a safe place to pull over and investigate. And if it’s a blown compressor belt, will you have one on board? This is obviously as important as a spare water pump/fan belt, and even more so than an alternator belt. So you’ll want to be sure to have one of each of these with you when traveling.


This isn’t the full air brake course, but hopefully it will help you decide what you need to know, and get you on to signing up for that air brake license and endorsement. You need to find out all you can about air brakes, their maintenance, checking and adjustment. Ask a trucker friend, get an Air Brake Manual at your local DMV and read it till you know it. It’s not really very complicated, especially on single-axle non-trailer vehicles. This little treatise is my attempt to get you “up to speed,” and is primarily designed to motivate you to get the whole course somewhere.

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