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An Air Brake Primer

An Air Brake Primer

- by Pete Snidal (C)2001 - revised Nov 2004

This little text file is an excerpt from 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 it may be better than nothing.

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.

1. The Differences

The first and most major difference is the complete lack of "feel" in the brake pedal, a feature we all become accustomed to with the otherwise much inferior hydraulic variety.

Air - A Valve, Not A Pump

The main and majorly important difference between air and the "normal" hydraulic brakes with which all are familiar, is in the fact that the air brake pedal is not a Pump, as with hydraulic brakes, but rather a Valve, which is entirely different. Operation of the brake pedal - correctly called the Treadle Valve - admits air to the brake chambers, which applies the brakes. Ther linkage between the air chamber and the brake shoes is external, to permit regular and frequent checking and adjustment.

With hydraulic brakes, if your linings are worn, and therefore getting too far away from the drums (need adjustment), you get a "low" pedal, which can be "pumped up" - you can actually correct the bad adjustment by pumping more fluid into the system, which will move the brake shoes closer to the drums. Not so with Air Brakes! - the pedal is simply a valve which allows air flow to the brake chambers. Once the chambers are full, they've travelled their full extent, and if the shoes are still too far from the drums, there's NOTHNG that can be done from the cab! 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.

One At A Time

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. So now, you're stopping with one less brake than you realize - in the light applications you use while driving normally, you might not even notice the difference.

A Wear Spiral

But 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!

The Air Supply

Another problem which crops up as the brakes wear is that as more movement is required on the part of the brake pots due to wear, more actual air is needed for each application, meaning your compressor is called upon to work harder. So we can agree, I'm sure, that keeping your brakes properly adjusted is very necessary! Uncorrected, the wear spiral will go on until your last brake's adjustment goes to nothing, and then you have, not Hydraulic Brakes, not Air Brakes, but 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 needs to be gotten around. (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 certain what 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, these will also suffice. But you can't leave an air maxi-brake (spring brake) on and adjust the rears - more on this later.

The Single-Pot Brake In The Off Position.
Note that both the shaft clevis on
the shaft and the pushrod length
are adjustable.
Tightening the worm adjusting nut rotates the clevis about the cam shaft, taking out the slack. Note the two adjustments.
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!
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.

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", it's time to tighten the worm nut and re-check, repeating the process until the adjustment is correct.

How Often?

Legally, alll trucks must stop at every trucks-stop-check brakes area and check slack. 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(ette) 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!

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.

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.)

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

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

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 the deapplication pressure is provided 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 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. 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.

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 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.

The air compressor must disconnect (it has a governor arrangement) at the top pressure - 120 psi in my case. 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. It also 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:

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 moreso than an alternator belt. So you'll want to be sure to have one of each of these with you when travelling.

The Low Air Indicators

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

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 condensateion, 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.

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 licence and endorsement.

Pete Snidal, Grand Forks BC Canada

You need to find out 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.