ASE
Brakes (A5)
A vehicle has an intermittent spongy brake pedal after driving on the highway for 30 minutes. An inspection shows that a newly installed rigid brake line is contacting an exhaust pipe heat shield. Which of the following explains this condition?
- AThermal expansion causes the proportioning valve to stick in the open position
- BHeat transfer creates a high-pressure hydraulic restriction in the line
- CHeat transfer causes the brake fluid to boil and create vapor pockets
- DExcessive heat causes the steel line to soften and balloon under pressure
Not A. Heat at the brake line does not cause the proportioning valve to stick open or explain a spongy pedal.
C is right. Brake fluid absorbs heat when routed too close to exhaust components, which can exceed the fluid boiling point and produce vapor pockets; because vapor is compressible, this results in a low, spongy pedal. Steel brake lines do not soften enough from exhaust heat to balloon under standard operating pressures.