CDL Air Brakes TestAir Brake System PartsMedium
A driver is operating a commercial vehicle and notices that the brake pedal requires significantly more travel than usual to achieve effective braking. The air pressure gauges indicate normal operating pressure. What is the most probable cause?
- AA miscalibrated supply pressure gauge.
- BA failing air dryer purge valve.
- CA restriction in the air compressor intake.
- DExcessive slack in the foundation brakes.
Show answer & explanationAnswer & explanation
Correct answer: D. Excessive slack in the foundation brakes.
If the brake pedal requires more travel for effective braking, and air pressure is normal, it suggests that the force from the brake chamber isn't being efficiently transferred to the brake shoes. Excessive slack in the foundation brakes (e.g., improperly adjusted slack adjusters or worn components) would cause the pushrod to travel further before the shoes contact the drum, resulting in a 'long' pedal.
Why the other options are wrong
- A. A miscalibrated gauge would show incorrect pressure, but if actual pressure is normal, it wouldn't cause excessive pedal travel.
- B. A failing air dryer purge valve would affect moisture removal, not the mechanical action of the service brakes.
- C. A restricted air intake would affect the compressor's ability to build pressure, not the pedal travel once pressure is available.
Excessive Brake Pedal Travel
A condition where the brake pedal must be pushed down further than normal to engage the service brakes effectively, despite adequate air pressure.
- Often indicates excessive slack in the foundation brakes.
- Can be caused by improperly adjusted slack adjusters or worn brake components.
- Reduces braking efficiency and increases stopping distance.
Memory trick: Long Pedal, Long Slack, Long Stop.