ASE G1 Auto Maintenance and Light RepairBrakesMedium
A technician is replacing the front disc brake pads on a vehicle. After installing the new pads and compressing the caliper pistons, they find that the brake pedal is low and requires several pumps to become firm. There are no visible leaks. What is the MOST likely reason for the low pedal?
- AThe master cylinder has failed internally.
- BThe caliper pistons were not fully seated against the new pads.
- CAir has entered the brake system during the pad replacement.
- DThe new brake pads are too thick for the caliper.
Show answer & explanationAnswer & explanation
Correct answer: B. The caliper pistons were not fully seated against the new pads.
After installing new pads, the caliper pistons need to be pumped out to fully contact the new, thicker pads. If this isn't done, the first few pedal applications will simply push the pistons out, resulting in a low pedal until they make contact and pressure builds. Air entering the system is less likely during a simple pad replacement unless a bleeder was opened.
Why the other options are wrong
- A. A master cylinder internal failure typically results in a sinking pedal, or a pedal that requires multiple pumps to build pressure, but it's less likely to suddenly occur *because* of a pad replacement without other symptoms.
- C. Air typically enters the system if a hydraulic line is opened or if the master cylinder runs dry. A standard pad replacement, without opening bleeder valves or lines, should not introduce air.
- D. New brake pads are designed to fit the caliper, and being 'too thick' would prevent installation or cause immediate binding, not a low pedal after installation.
Brake Pad Seating
After replacing brake pads, it's crucial to pump the brake pedal several times to push the caliper pistons out and seat the new pads against the rotors, restoring proper pedal height and feel.
- Piston retraction for new pads
- Initial low pedal is normal
- Pump pedal to seat pads
Memory trick: New pads, low pedal? Pump it 'til it's settled!