Low air pressure with a full load
At a quarry a 26 t tipper is loaded with 18 t of gravel. The driver starts the engine and sees the air-pressure gauge at 5.5 bar; the low-pressure warning lamp and buzzer are on. The compressor is running, and there is an audible hiss from the rear-left spring-brake chamber. The service brake pedal feels normal at first press, but the gauge drops to 5.0 bar when the pedal is held. The exit is a 2 km gravel road down to a public road with a 6% descent. The driver has a 15 min window before the quarry closes and says he can build pressure on the move. The lorry has a working tachograph, wheel chocks and a warning triangle.
1. Interpret the symptoms and decide whether the lorry may be moved.
Read the worked answer
The normal operating pressure for this type of air-brake system is roughly 8 to 10 bar, so 5.5 bar is already low and the warning is correctly active. The pressure dropping further to 5.0 bar when the pedal is held shows that the system cannot maintain pressure under demand. A continuous hiss at a rear spring-brake chamber points to a leaking chamber, diaphragm or line. That can affect both service braking and parking-brake release, and it can cause a spring brake to drag or fail to hold. The lorry must not be moved on the public road or down the 6% descent. Building pressure on the move is not a valid repair; the leak will continue and the descent will increase the demand. The correct decision is to park safely, chock the wheels and call the workshop.
2. What immediate checks and actions should be taken?
Read the worked answer
The driver should apply the parking brake, chock the wheels, switch off the engine and put the gearbox in neutral or park according to the vehicle. The area around the leaking chamber should not be entered unless the vehicle is securely supported, because air suspension or spring brakes can move. The driver should look for damaged air lines, a split diaphragm, loose fittings or ice in the system, but should not crawl under an unsecured vehicle. The defect should be reported to the operator and recorded in the daily defect report. The load should remain on the vehicle if it is safe, and another vehicle or a workshop should be called. The warning triangle is not a substitute for repairing the brake system.
3. If the gauge had been 8.5 bar and the hiss appeared only after the parking brake was released, would that change the decision?
Read the worked answer
It would change the urgency but not the need to investigate. A constant hiss from a spring-brake chamber after release can still indicate a leak or a sticking valve. The driver should perform a stationary leak test: build full pressure, switch off the engine, release the parking brake, hold the service brake and watch the gauge for a continuous drop. Any significant continuous drop, or a hiss that does not stop, means the vehicle should not be driven. A brief purge from the air dryer is normal; a hiss at a brake chamber is not. With 18 t of gravel and a 6% descent, the margin for a brake defect is too small to accept.