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.
|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.
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!)
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.
Note that both the shaft clevis on
the shaft and the pushrod length
|Tightening the worm adjusting nut rotates the clevis about the cam shaft, taking out the slack. Note the two adjustments.||
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!
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.
(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!
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.
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:
You need to be advised when your air pressure drops below a safe level. The means of advising you are"
- 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.
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.)
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!
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.