CDL Air Brakes TestAir Brake System PartsMedium
A technician is diagnosing an issue with a truck's air brake system where the brake pedal feels spongy and requires excessive travel to achieve braking. The supply pressure gauges indicate normal operating pressure. Which component is the most likely cause?
- AA malfunctioning safety valve.
- BA faulty air compressor governor.
- CExcessive slack in the foundation brakes.
- DA clogged alcohol evaporator.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive slack in the foundation brakes.
A spongy pedal with excessive travel, despite normal air pressure, points to an issue with the mechanical linkage or adjustment of the foundation brakes. Excessive slack means the brake shoes/pads have to travel further before contacting the drum/rotor, leading to a 'spongy' feel and requiring more pedal travel.
Why the other options are wrong
- A. A malfunctioning safety valve would cause air leaks or over-pressurization, not a spongy pedal with normal pressure.
- B. A faulty governor would affect air pressure regulation, not pedal feel or travel directly if pressure is normal.
- D. A clogged alcohol evaporator would affect moisture removal in cold weather, not direct pedal feel or travel.
Foundation Brakes
Foundation brakes are the components at each wheel that perform the actual braking, including brake drums/rotors, shoes/pads, and cam shafts/calipers, actuated by air pressure.
- Convert air pressure into mechanical braking force.
- Must be properly adjusted (slack adjusters).
- Include drums, shoes, linings, and chambers.
Memory trick: Pedal travel tells brake health.