Air Brake System Failures and Emergencies
This domain tests what you do when pressure is lost, a circuit fails, or the brakes overheat - and which automatic protection you can actually count on.
Losing air
- The warning must come before 60 psi. When it does, stop safely: you have only a few applications left, and the spring brakes will set on their own somewhere between 20 and 45 psi.
- You can use the parking brake valve (yellow knob) gradually to help stop, but it is a spring-brake control, not a service brake, and a full application can lock the wheels.
A failed circuit
- Dual systems are split, for example tractor/trailer or front/rear. One failure leaves the other circuit working, but stopping distance grows and air may keep escaping. Watch for a fast-falling gauge, a hiss, or a pull - then park it.
Brake fade
- Hot drums and linings lose friction. The pedal can feel firm while the truck will not slow. Causes are riding the brakes down a grade or repeated high-speed stops. Response: downshift, use the engine brake or retarder, and snub - firm application, then off to let them cool.
Breakaway protection
- If a trailer separates, the tractor protection valve closes, the trailer emergency relay valve releases its air, and the trailer spring brakes set. The trailer air supply knob pops out.
Worked example
On a long grade the driver smells hot brakes and the truck will not slow, but the gauge holds normal pressure. Normal pressure plus no retardation means fade, not an air leak. The fix is gears, engine braking and cooling - not pumping the pedal, which only adds heat.
Traps
- Pumping the brakes during fade.
- Believing the spring brakes apply only at 0 psi; they set by 45 psi.
- Treating a low-pressure warning as permission to drive to the shop.
- Using the trailer hand valve as a parking brake.
- Expecting the tractor protection valve to stop the tractor.