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.
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.