CDL Air Brakes TestAir Brake System PartsHard
During a road test, a driver finds that the service brakes are slow to release, causing the vehicle to drag slightly. The air pressure gauges indicate normal operating pressure. Which component is the most likely cause?
- ARestricted exhaust ports in the brake pedal (foot) valve.
- BA malfunctioning low air pressure warning switch.
- CA faulty air compressor.
- DA stuck-open safety valve.
Show answer & explanationAnswer & explanation
Correct answer: A. Restricted exhaust ports in the brake pedal (foot) valve.
If the service brakes are slow to release despite normal air pressure, it indicates that the air pressure is not being exhausted quickly enough from the brake chambers. The brake pedal (foot) valve controls both the application and release of air to the service brakes, and restricted exhaust ports would trap air, causing slow release.
Why the other options are wrong
- B. A malfunctioning low air pressure warning switch affects only the warning, not the brake release mechanism.
- C. A faulty air compressor would lead to low or no air pressure, not slow release with normal pressure.
- D. A stuck-open safety valve would cause air loss, not slow brake release with normal operating pressure.
Brake Pedal (Foot) Valve
The brake pedal (foot) valve, also known as the treadle valve, controls the flow of air from the air tanks to the service brake chambers, both applying and releasing the brakes proportionally to pedal pressure.
- Sends air to both primary and secondary brake systems.
- Contains exhaust ports for brake release.
- Proportional control: more pedal, more air pressure.
Memory trick: Quick exhaust, quick release.