A technician is performing a brake system flush. After flushing the system, a pressure test reveals that the residual pressure valve in the master cylinder is stuck open. What effect would this have on the disc brake system?
- AA spongy brake pedal and excessive pedal travel.
- BDragging brakes and premature pad wear.
- CDelayed brake engagement and increased pedal effort.
- DA hard brake pedal with very little travel.
Show answer & explanationAnswer & explanation
Correct answer: A. A spongy brake pedal and excessive pedal travel.
The residual pressure valve in a disc brake master cylinder maintains a slight pressure (typically 0-2 psi) in the brake lines to keep the caliper piston seals expanded and ready for immediate engagement. If this valve is stuck open, it won't maintain this slight pressure, allowing the piston seals to relax and potentially requiring more pedal travel to take up the slack before the pads contact the rotor, resulting in a spongy pedal.
Why the other options are wrong
- B. Dragging brakes would occur if the residual pressure valve was stuck *closed*, maintaining excessive pressure in the lines, not stuck open.
- C. Delayed engagement and increased pedal effort are symptoms that could arise, but 'spongy pedal' directly describes the feeling of taking up excessive slack, which is the primary result.
- D. A hard pedal with little travel typically indicates a restriction in the system or a seized component, not an open residual pressure valve.
Residual Pressure Valve (Disc Brakes)
The residual pressure valve in a master cylinder maintains a slight pressure (0-2 psi) in disc brake lines to keep caliper piston seals expanded, ensuring immediate pad contact.
- Prevents excessive piston/pad retraction.
- Stuck open: spongy pedal, increased travel.
- Stuck closed: dragging brakes.
Memory trick: Residual valve open, pedal's slow to know, where the pads need to go!