Article: The Brake System – Adding Spring Brakes

Adding Spring Brakes To Your System

By Pete Snidal ©2004

In the early days of air brakes, only "service Brakes" - the on-the-road brakes - were provided for. Parking and/or "emergency" brakes were supplied by a drum or disk brake mounted on the drive shaft. Since total braking capacity is soon overwhelmed if heat buildup is allowed to occur, and since increasing total brake area is the major way to overcome this, it's best to use the rear service brakes as parking/emergency brake.

The only difficulty with this scheme is actuation - how can the air-operated rear brakes be applied in the event of loss of air pressure (emergency) or for parking, when the air supply eventually depletes due to the air compressor being shut down with the engine (parking)?

Enter the Spring Brake. These utilize a second system of brake actuation - only for the rear brakes - being applied by very strong springs, which duplicate the action of the otherwise air-operated brake chambers, by the addition of "Spring Chambers," which are "piggy-backed" onto the former. This system, although not originally supplied on the Flx Clippers, is a far superior and much safer one, and thankfully may fairly easily be retrofitted to the Clippers. The job is in fact only a few hours' work, and requires less than $200.00 worth of parts and pieces. And, best of all, you can leave your original driveshaft brake in place for extra backup!

Do You Already Have Spring Brakes?

Before we panic, let's check and see if you already have spring brakes. This may be determined by looking in two places: your driver's control panel, and the rear axle.

  • A. The Dash - The spring brake control valve, usually labelled "maxi brakes," "parking brake," or some such, will be a plumbing element designed to admit air back to the spring brake chambers. In the "off" position, it will be open, allowing air (once the reservoir pressure is sufficient) to the brake chambers, compressing the springs. Closing the valve will open the passage between it and the chambers to the atmosphere, allowing the air to bleed from the chambers, and the springs to engage and apply the rear brakes - the "on" position.
  • B. The Rear Axle - in either event, you will have a service brake chamber at each side of your rear axle. If these chambers are double, ie there is a second chamber "piggy-backed" onto the first, then you have spring brakes. If you needed to read this paragraph to know, you are not equipped to be driving an air-brake-equipped vehicle, and you need to find out a lot more about them. Start by reading the general article on air brakes.

If your chambers are still single, you should have a knowledgeable shop or mechanic install spring brakes on your rig. If you're not already confident in your competence to do this job, do not consider this article a "how-to." Rather, it is intended to acquaint you with what is involved with the job.

First, let's look at Spring Brakes in detail. Following is an excerpt from an earlier article on Air Brakes in general:

Spring Brakes - Parking Brakes for Air-Equipped Vehicles

Since it would be ridiculous to expect an air chamber to maintain pressure over long periods of parking, the spring brake has just that - a spring applies the rear brakes, which are therefore always on, unless the spring is "caged" by a 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.)

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!

Important Safety Note!

Does this mean that you will always have an automatic emergency brake, which will come on every time you lose pressure? No, there is still one eventuality which will not - ever - be accounted for, and that is excessive adjuster slack! The final determinant in the airbrake equation is always the mechanical linkage between the air chamber/brake pot and the actuation lever - the slack adjuster. If any adjuster - or all of them - is too slack, no amount of spring or air pressure is going to put that brake on - the push rod will move to its full extent, and 20,000 Lbs of coach will sail blithely on into whatever disaster awaits. NEVER let your adjusters get too slack! Check them daily, and more than that when you're using your brakes more than usual.

And don't ever forget, the rear brake slack must be done with the brake springs caged! - So you'll have to chock your wheels and set your brake control to the "off" position.


Since this is a vital part of the safety considerations for operating your coach, a certain level of mechanical expertise is required, and certain standards must be maintained during this installation. If in doubt, enlist the services of a certified mechanic, or take your Clipper to a reputable shop to have the installation done. Still, it may be a help to read on, if for no other reason, just to understand what will be involved in the procedure.

First, the Caveats. A little discussion of what could go wrong:

  • 1. Sudden release of spring brake pressure

    Imagine driving along on the highway, traffic behind you fairly heavy, and perhaps following a little too close, when suddenly, without warning, _your_rear_brakes_lock_up! The possiblities for bad craziness are manifold. First, you may be involved in being the star player in a rear-end collision - or even a multi-car pileup! You can imagine the repurcussions when the insurers discover that your homemade band-aid and clothespins spring brake conversion let go!

    Assuming that didn't happen (thank gods), you're still stuck with your emergency lights flashing, in the middle of the lane. You've got to get your coach out of the way as soon as possible, meanwhile first setting out flares and directing traffic around this obstruction you've created. (With luck, you've got some crew aboard to help with some of this.) Your brakes can be released, fortunately, by using your "caging bolts" to withdraw the brake springs. It means getting out and under, and some sweaty work with a big wrench for 10 or 15 minutes. Hopefully, you've set blocks under your wheels, and set your old parking brake (if it's still there), so your bus doesn't roll over you when you get the brakes off, and you can then close your parking brake valve - to eliminate any more air loss - and drive to somewhere where you can diagnose and repair the problem sans your parking/emergency spring brake. (Which is hopefully backed up by your still-there driveshaft brake. Belt and suspenders.)

  • 2. Development Of A Sudden Air Leak

    With this one, your low air light and buzzer suddenly come on - or, better yet, you noticed your pressure gauge dropping very fast and without warning, or maybe (unlikely) you even heard some air hissing and then looked at the gauge. This may have been the result of your retrofitted spring brake system leaking air somewhere between the take-off point (from your existing system) and the brake chambers, or it could be from some other cause. In any event, you're losing air pressure, and this means you have to get that puppy over to the side of the road before the brake springs take over due to lack of sufficient air pressure in the de-application chambers. About 40 psi is where it starts to happen. This scenario is not nearly as scary as the first, depending of course on how rapidly the air is escaping, and how soon you became aware of the problem, but it can still give cause for wishing your new plumbing had been made a lot more secure.

Either of these scenarios assumes the main possible problem with your installation - that of air loss from your new line letting go. So we see right away that this new airline - from take-off point through control valve to brake chambers - as with all brake components - MUST be reliable and totally secure!

Other possible problems could develop from faulty installation of the new brake chambers themselves - failure to tighten the mounting bolts properly, install safety cotter pins in the linkage clevises, or possibly failure to adjust the rear brakes properly after installation - remember, your rear brake adjustment must now be done with the brake springs caged by air pressure or bolt installation!

However, whether you decide to do the job yourself, or "farm it out," here are the steps involved:

Doing The Retrofit

First, you'll need to assemble the necessary parts and pieces. Main ones are:

  • A pair of new "Spring," or "Maxi-Brake" rear brake chambers.
  • A Plumbing Tee Fitting which can be mounted on the rear cross-member of your frame, just ahead of the rear axle.
  • Flexible Lines to run from the tee to the Spring Brake Pots
  • A Line and the required fittings to run from the tee forward to the driving position - to a dash or even floor-mounted valve which will be used to charge the spring brake chambers - or to release the air pressure to them to allow the springs to set the rear brakes for parking or emergency.
  • A valve to connect the charging line to the air system
  • A tee to tie in an air supply to your new springbrake control valve

A Word On Pipe And Line Fittings

There are a number of types and standards of what most people just refer to as "pipe fittings." They vary in how they seal, thread pitch and diameter, and purpose. The different main types are not interchangeable, and MUST not be confused with one another. To go from one type to another requires adapterfittings in between. (eg, pipe thread to tubing flare.) The basic types you will encounter are:

  • 1. Pipe Thread - these are connected and sealed by means of tapered threads - ie, the diameter of the external threads gets larger as the fittings screw together, and this, in conjunction with pipe compound or sealing tape, seals the joint. This type of sealing is used with fitting-to-fitting (eg elbow to union) connections, and at the ends of straight, rigid pipes - seldom used where vibration is present.
  • Flare Fittings - these are used to seal the ends of runs of tubing. A flare nut is slipped over the end of the tubing, then a flaring tool is used to flare out the end of tube, which is then clamped over the reverse of the flare on the flare fitting by the nut. The other end of the flare fitting may also be flare, or it may be an adapter fitting, the other end (or ends, in the case of a Tee or 4-way) being of a different sealing type, such as Pipe or Compression.
  • Compression Fittings - these are also used for tubing. Rather than a flare on the line being clamped down onto a fitting, a special bushing nutis slipped over the end of the tube, followed by a special bushing. The bushing is tapered on each end, with a hole the exact size of the tubing through its center. The bushing nut and the compression fitting have reverse tapers inside them which meet the tapered ends of the bushing. The nut is screwed against the fitting, compressing the bushing, which squeezes the tubing to make a seal.Tubing may be of soft (semi-rigid) copper, or plastic. For air work of any kind, always ensure that you are using the proper schedule (a rating of pressure and quality) for the job at hand. Different grade tubing is used for aquariums and Pepsi-Cola dispensers than for Air Brakes!

    There is a special type of compression fitting for hi-grade plastic tubing, approved for airbrake use, which uses a special insert which is slipped inside the tube before the nut is screwed to the fitting. This reinforces the end of the plastic tube sufficiently for a solid compression joint to be made.

    Often, the compression bushing forces its way into the tubing, so although the joint may be connected and reconnected repeatedly, bushings should not be reused for other connections. New bushings should be used each time, and the tubing recut to a fresh surface.

To reiterate, adapterfittings must be used to connect joining methods of different kinds together. Thus a NPT (National Pipe Thread) Tee may be connected to plastic or soft copper tubing, providing a fitting is used which is NPT on the tee end, and Compression or Flare on the other (depending on what you decide to use on the tubing.)

Doing The Retrofit

1. The New Brake Chambers

First, secure your new brake chambers. You'll want series 30dual spring and service brake chambers. The "30" is the size - refers to the area of the diaphragm. Fortunately, the bolt patterns for mounting are standard. The push rod length varies, but your new universal-fit chambes will have pushrods that are too long, and threaded for their entire length, so you'll be able to cut them down to fit. They may be ordered at any truck parts outlet. If in doubt, take one of your old ones in so the counterman can get any dimensions he may need.

The next step is to remove the first of your existing brake chambers. Before you do this, check that the existing pushrod length is correct. To do this, use a Brake Buddy(TM) or other suitable tool to move the slack adjuster to full brake position. Check the angle between the pushrod and the slack adjuster lever at this point. If the angle has gone to less than 90 degrees (this is a natural occurence due to brake shoe wear), you'll want to lengthen the pushrod by adjusting the position of the clevis - ie, make your new pushrod just a bit longer. Remember this for when you set the length of your new pushrod. Then, locate the clevis pin at the outer end of the slack adjuster. Remove the cotter key from the end of the pin, and then remove the pin. Now you're ready to detach the air line between the chamber and the distribution tee - look for the swivel fitting and unscrew this first. (One end will likely be a "swivel" fitting, and the other will be a pipe fitting that requires twisting the whole line to move.) Then you can unscrew the two big nuts that hold the brake pot onto the bracket welded to the rear axle, - these are fun; one of those 1/8 turn at a time propositions. Once you get them off, you'll be able to lift the pot free of the axle. Now for some bench work.

Once you've got the first brake chamber on the bench, you can use it to determine where to cut the pushrod supplied with the first of your new double-pot chambers. - They come in a universal length, to be cut for the particular application of the new owner.

First, measure the distance from the mounting face of your old pot to the center of the clevis pin holes. Make a note of this distance - the target is to cut the pushrod on the new double pot, and to reinstall the clevis so the distance is the same.

Now, examine one of your new double pots. There is an air chamber at the rear, just as with your old one, but there is an additional air chamber "piggy-backed" to this one. The latter chamber contains a very strong and highly compressed spring - DO NOT EVER under any circumstances contemplate removing the retaining ring which holds this spring chamber together. This WOULD result in heavy damage to your person! - that spring is under STRONG tension! Disassembling Spring Brake Chambers without the proper tools fits in with taking hammers to old TV tubes as a qualifier for the Darwin Award.

And soon it will be under even more tension, since the spring brake must be de-activated before we continue with the installation. This will mean using the cage bolt to hold the spring tensioned, once we get it that way. There are two ways of tensioning the spring. The first is just to fit the cage bolt into the end of the chamber - it will rotate and lock into place inside the spring retainer - and use a suitable wrench to tighten the bolt, drawing the spring up against the end of the chamber. This starts out to be a fairly easy chore, but as the spring tension increases, the torque required increases substantially - the nut will actually get quite hot, as will you yourself. It can be quite a workout.

Or you can use shop air pressure, if you have it available, to do the work for you - this is easier on the nut and threads, and much easier on your arm. Chuck the air chamber securely in your bench vise, and remove the protective plug in the spring air chamber air fitting. Then fit your blow gun into the air fitting, and apply air pressure. This will force the spring up against the end of the chamber, releasing tension on the bolt. This will allow you to run the nut down the threads with much less effort - you'll need 30-40 psi to pull the spring completely back. DO NOT get your fingers between the nut and washer, or washer and chamber - if the blow gun slips out of the fitting, spring pressure will suddenly return with a vengeance!

Once you have the spring completely caged, the service brake chamber will operate just as did your old one, and you may continue with the installation. First, returning to the original air chamber, loosen off the locknut and unscrew the clevis. Now, you'll want to cut the new pushrod so that you can screw on the old locknut and clevis to attain the same effective length (mounting flange to clevis length) as your old one. (This is the time to adjust the length for that 90 degree angle if necessary!) Do the measurements and then make the cut - with hacksaw or Skilsaw and cutoff disk - and screw on the locknut and the clevis - you may have to dress the end of the pushrod with a fine file, depending on how violent the cutting process was. Be sure to tighten the locknut securely against the clevis.

This done, you'll be able to mount your new dual brakepot in place of your old one. Remove the new mounting nuts, and introduce the new chamber to the flange on your rear axle. Re-fit the clevis pin while you can still move the chamber around on the flange, and then fit the lockwashers and mounting nuts. Snug them up, and then, using your brake buddy or similar tool, pull the slack adjuster against the brakes, and check that the angle between pushrod and slack adjuster at this point of application is a bit more than 90 degrees. If it is less than 90, you'll have to adjust the pushrod length by means of the clevis until it is less than 90 - as the brakes wear, this angle will increase past the ideal 90 degrees until it is more, and you'll have to adjust it again. Thankfully, this is not a daily chore, like adjusting the slack adjuster. Once you've got the angle right, (no pun intended) be sure to secure the clevis pin with a new cotter key. Ensure that the pot mounting nuts are now tightened securely.

Now, adjust the slack adjuster for 1/4" or a bit less of free play - meaning so that you can move the end of the adjuster only this much before the movement is stopped by the shoes butting against the inside of the brake drum. This adjustment is done by rotating the 9/16" bolt head which rotates the worm in the slack adjuster lever. Tighten the bolt until you can get no movement, then back it off until you can move the pushrod about 1/4".

You may now re-fit the service brake air line to your new service brake chamber. You will likely need a new fitting or two - a 45 degree will likely be required to re-orient the line for the new fitting position on the air chamber. Use high-quality brass fittings, obtainable at the truck supply house where you got the air chambers and air lines. Resist any temptation to use anything but the highest-quality fittings suitable for airbrake use. If in doubt, ask the parts man! Be sure to route all brake hoses so that they won't be able to snag on any objects projecting from the roadway.

This done, repeat for the other side. Once the brakes are re-connected, you have as much brakes as you did before, with a pair of new, still-caged spring pots which give you the potential to continue with spring brake installation. All that's left to do is to provide for a driver-controlled air supply to the spring brake chambers, and to air them up and remove the caging bolts.

2. The Air Supply

A. The Distribution Tee And Air Lines

Now, you'll want to mount your new air supply tee on the cross member just ahead of the rear axle. Mount the tee beside the existing one for supply air. Fit the pipe thread-air hose flare adapters, angle fittings, and your new flexible air hoses as required, connecting the air chamber fittings to the tee. This tee may be a standard 1/2" pipe tee, with a mounting bracket brazed to it, or your parts supplier may be able to come up with something more exotic. Just get a size that will allow direct fitting of your brake hoses. You'll likely need an adapter for the air line from up front.

Now, to provide air supply to the tee. First, decide on your routing for the air line from tee to the valve position at the driving position. DO NOT even contemplate running the line below any cross-members - if this line catches on any obstructions and tears out, your rear wheels will lock up immediately! You'll likely have to drill a hole in each cross-member for your new line. Obtain the proper fittings to adapt from the thread type of your tee to the fitting type of your chosen line - you can use soft copper of the proper schedule (thickness and psi rating) as was used on your original air system, but you'll likely chose the modern air-brake approved plastic line and matching ferrule-and-filler end fittings. You'll likely need adapter fittings from this type of fitting to the threads of your tee, and to those of your control valve at the driver end. (See earlier discussion of types of threads and fittings.) I used 3/8" line; this is a minimum size; you may chose to use 1/2". (Remember a high-capacity, reliable air supply is essential to safe operation of your coach with spring brakes.)

When you purchase your line, be sure also to buy rubber grommets to mount where the line goes through the cross-members, or any other metal parts of the coach. This is to inhibit chafing of the plastic, another possible cause of catastrophic unwanted brake application. The grommets will tell you what size to drill your holes.

Once the holes are drilled, route the line from the tee to the driving position - you'll likely want to mount your control valve on the dashboard.

B. The Control Valve

There are a number of possibilities for control valves. See your local supplier for options, and choose the kind of valve you want - of course, you'll be using one designed specifically for the purpose. The other consideration is where to mount it - generally on the dash, and from where you'll be drawing the air supply to feed it.

This last one can be perplexing. The original Flxible air system, using semi-rigid copper tubing, presents a number of problems to break into a place such as the supply to the treadle valve, or even the air horn button on the floor. After some investigation, your writer chose the supply to the air pressure gauge. This is not a high-volume connection, since the air gauge supply is only 1/4" line. However, my reasoning is that connecting into the air gauge supply close to the gauge means that any leakage from the new tee downstream to the springbrake pots will show in the gauge quickly, and I'll just have to monitor and be ready for it. So I put a tee in the back of the pressure gauge, and connected the existing line to one side, and the input to my springbrake valve on the other. The line from the springbrake valve to the distribution tee at the rear axle is 3/8".

Once everything was connected, I started the engine, and when pressure was up to snuff, opened the springbrake control valve. This pressurized the springbrake chambers, taking the pressure off the caging bolts, which I was then able to remove as finger-tight. I was, of course, careful to set the mechanical parking brake before climbing under there and pulling out the cabe bolts, since once that is done, the vehicle is otherwise free to roll. Releasing the valve, of course, sets the brakes.

And that, boys and girls, is how I installed spring brake pots on my Visicoach. Let me reiterate - if you are in any doubt whatsoever as to your competence with the theory, the plumbing, or the mechanical considerations of the job, have it done by a professional - or, at least, checked by one as you complete the various stages. Brakes are the very last thing you want to take chances with!

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