Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is evaluating a vehicle that exhibits a 'hard pedal' and significantly reduced braking effectiveness. The brake booster vacuum hose is properly connected and shows no signs of leaks. When the engine is running, the technician measures the vacuum at the booster inlet at 18 in. Hg. With the engine off, the pedal is depressed and held, then the engine is started. The pedal does NOT drop. What is the MOST likely cause?
- AA restricted brake line.
- BAir trapped in the hydraulic system.
- CA faulty brake booster diaphragm.
- DAn internal master cylinder leak.
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty brake booster diaphragm.
The 'pedal drop' test is a common way to check booster function. If the pedal does not drop when the engine starts (indicating vacuum assistance), and vacuum is present at the booster inlet (18 in. Hg is good), it strongly suggests an internal failure of the brake booster diaphragm or check valve, preventing vacuum from assisting the pedal.
Why the other options are wrong
- A. A restricted brake line would cause a hard pedal and reduced braking, but the pedal drop test would indicate a functional booster.
- B. Air in the hydraulic system would cause a spongy pedal, not a hard pedal.
- D. An internal master cylinder leak would cause a sinking pedal, not a hard pedal.
Brake Booster Pedal Drop Test
A diagnostic test for vacuum brake boosters where the pedal is held down with the engine off, then the engine is started. A functional booster will cause the pedal to drop slightly.
- Checks for proper vacuum assistance.
- Pedal should drop when engine starts.
- No drop indicates booster or vacuum issue.
Memory trick: Hard pedal, no drop, booster's a flop.