CDL General Knowledge TestAir Brakes (if applicable)Hard
A driver hears a constant, faint hissing sound coming from under the cab of their air brake equipped truck while parked with the engine off and brakes released. The air pressure gauges are slowly dropping. Which component is the most likely source of this leak?
- AA leaking parking brake control valve.
- BThe service brake relay valve.
- CThe air compressor discharge line.
- DA leaking brake chamber diaphragm.
Show answer & explanationAnswer & explanation
Correct answer: A. A leaking parking brake control valve.
With the engine off and brakes released, a constant hiss from under the cab, accompanied by a slow pressure drop, points to a leak in the parking brake system. The parking brake control valve (often located in the cab area) is a common point for such leaks when the parking brakes are released, as it controls the air flow to hold the parking brakes off.
Why the other options are wrong
- B. The service brake relay valve is active when the service brakes are applied; a leak here would be more apparent during service brake application.
- C. The air compressor discharge line would be under pressure when the engine is running and compressor is active, less likely a constant leak with engine off and brakes released.
- D. A leaking brake chamber diaphragm would typically be audible at the wheel end, and often more pronounced with the service brake applied or parking brake engaged (depending on type).
Parking Brake Control Valve Leak
A constant hiss from under the cab with engine off and parking brakes released, accompanied by a slow air pressure drop, often indicates a leak in the parking brake control valve.
- Located in/near the cab.
- Controls air to parking brakes.
- Leaks when parking brakes are released (air supplied).
- Causes slow system pressure loss.
Memory trick: Hiss from the cab, brakes released, parking valve is not at peace.