Maintaining Your Air Brake System
By Pete Snidal © 2004
Operation of each of the components of the Air Brake must be monitored at all times. In addition, a "Pre-Trip Inspection" should be carried out before every movement of your coach. This will check the operation and security of each of the elements in the system.
1. The Compressor
Air brake compressors function efficiently year in and year out for a long time between required servicings. Thus it is easy to take them for granted, but parts do wear, and their operation will degrade over time. The best way to monitor their performance is simply by observing how long they take to build up air when the vehicle is started with the air depleted. This is also the time to check the operation of the Air Pressure Regulator - does it "cut out" at the proper high limit, and "cut in" at the proper low one? Here's the procedure:
Upon starting the engine, the driver makes a note of the position of the second hand of his watch - or begins counting, and notes the time it takes for the air to build up to a certain value - the end of the gauge's "red zone," is a good one. The air pressure of the system, assuming it's starting at Zero, must reach the end of the "red" zone of the gauge - 80 psi - within 90 seconds. It must continue to build to 120 psi, according to BC DOT regulations, within 3 minutes. At this point - and this is important! - it must STOP building pressure - ie, the regulator must deactivate the compressor and the gauge will be seen to stop climbing. If it continues to climb, there is a fault in the regulator, and the engine must be shut down and the condition must be checked.
Once it has been established that the compressor is building air within the required time limit, and that the regulator is "cutting out" at the required pressure, the DOT-mandated Pre-Trip Check requires that the operator then reduce the air pressure by "fanning" the brake pedal repeatedly until the low limit is reached, and ensuring that the regulator "cuts in" at that pressure. With the Flxi's, it should cut in - ie, the gauge should begin once more to climb - at the top of the "red zone" - the 80 psi lower limit. Once again, the max pressure of 120 psi must be attained in a reasonable time period - within 2 minutes - and the compressor must once again "cut out."
During the period of brake fanning to reduce pressure, you should also observe just how much of a pressure drop you get for each application of the brakes. How hard the application is should make little difference - the air chambers fill with a 1/2" brake pedal movement just as much as when it's floored. (This should NEVER be done when the spring brakes are applied, by the way - more on this later.) A drop of about 5 psi per brake application is normal. Much more than this is a sign of possible leakage of a line or air chamber. If this occurs, the operator should have a helper apply some brake while he listens at each wheel and under the bus in general for the sound of any escaping air.
Once the compressor and its regulator have been checked and approved in this fashion, we may proceed with the rest of the system.
2. Reservoirs, Lines, And Brake Chambers
The engine should then be shut down, and a "walk-around" should be done to listen for any signs of escaping air. It is a good idea to have a helper apply brake as well - 1/2-3/4" of brake pedal is enough - while you listen for any sign of air escaping from an air chamber. Of course, if any air leak is detected, it must be investigated and repaired. Leaks may be checked for with soapy water applied with a brush to suspected joints and lines. Air escaping within a brake chamber indicates a leaking diaphragm, and this condition must be remedied before operating the vehicle. Exchange brake chambers are not expensive. If replacing a (double) (rear) Spring Brake chamber, remember NEVER to attempt to dismantle the chamber!
3. The Brake Slack
Now for the slack adjusters - these must be checked before every trip, and of course adjusted if necessary.
Before checking any brake adjusters, ALWAYS first ensure that the bus cannot roll over you - you'll be lying down very close to a wheel in every case. The spring brakes will have to be disengaged, since all slack is of course taken up when they're on. This means you'll have to block at least one wheel - fore AND aft! - even if on level ground - and more if on a slope, before disengaging the spring brakes. If you still have a driveshaft brake as well, it won't hurt to engage it, but it's not a good idea to count on this as your only safety, especially if it hasn't been tested recently. (It may be tested by attempting to move the bus with the handbrake set, and no blocks at wheels.)
Once the safety considerations have been met, it's time to check the slack at each adjuster. Truck supply houses sell a "Brake Buddy" (TM) - a special wrench with a fitting to allow it to be used as a lever on the slack adjuster. These are a handy thing to have in the tool box - with your tire pressure gauge and tire beater. If you don't have one, you'll have to move the slack adjuster some other way, but the object in any case is to move the slack adjuster at the pushrod end back and forth, checking the distance it moves at this point.
The slack should be set to 1/4." If it moves more than 1", it must be tightened. This is accomplished by rotating the adjuster worm, by means of the adjuster bolt, about the pinion gear until all slack is taken up, then backing off on the bolt until there is 1/4" of free play.