Heavy Vehicle Braking Systems and Air Brakes
Air brake questions are the most technical in the heavy-vehicle paper. Learn the components, the daily tests and what each gauge reading means.
How the system works
- A engine-driven compressor charges the reservoirs; the governor cuts out near 100 psi and cuts back in near 85 psi (check your vehicle's manual).
- The foot valve meters air to the brake chambers; pushrod stroke moves the slack adjusters and shoes.
- Spring brakes are held off by air pressure. Lose pressure and they apply automatically - that is why they are the parking brake.
- A low-pressure warning buzzer and lamp operate around 60 psi; spring brakes normally apply between roughly 20 and 45 psi.
Daily tests
- Build full pressure, switch the engine off, hold the foot brake for three minutes: loss should be within the manufacturer's limit (typically about 2 psi/min for a rigid, 3 psi/min for a combination).
- Pump the pedal down to check the warning activates and the spring brakes apply.
- Drain all reservoirs daily - condensation is constant in Jordan's temperature swings.
Worked example
On a descent the low-pressure buzzer sounds and the gauge reads 55 psi and falling; the vehicle slows on its own. Reasoning: the spring brakes are applying as pressure drops. Do not fight it or release the parking valve repeatedly. Steer to a safe stopping place, let the vehicle stop, then find the leak, and do not continue until pressure builds and holds.
Traps that cost marks
Testing with the engine running so the compressor masks a leak; fanning the pedal downhill instead of snubbing; using the parking brake as a service brake.