Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is performing a diagnostic on a vehicle that intermittently exhibits a 'hard' brake pedal, but only when the Anti-lock Braking System (ABS) is actively engaging (e.g., during hard braking on a slippery surface). Conventional braking feels normal otherwise. What component is MOST likely at fault?
- AA collapsed flexible brake hose.
- BThe ABS hydraulic control unit (HCU) pump motor.
- CThe brake booster check valve.
- DThe master cylinder primary piston seal.
Show answer & explanationAnswer & explanation
Correct answer: B. The ABS hydraulic control unit (HCU) pump motor.
If the hard pedal occurs specifically during ABS activation, it points to a problem with the ABS HCU. The pump motor in the HCU is responsible for rapidly returning fluid to the master cylinder and maintaining pressure during ABS events. If it fails, the HCU cannot properly modulate pressure, leading to a hard pedal feel during ABS operation.
Why the other options are wrong
- A. A collapsed flexible brake hose would cause a dragging brake on one wheel, not an intermittent hard pedal during ABS events.
- C. A faulty brake booster check valve would cause a hard pedal all the time, not just during ABS activation.
- D. A master cylinder primary piston seal issue would cause a sinking pedal, not a hard pedal during ABS activation.
ABS HCU Pump Motor Failure
A failing ABS Hydraulic Control Unit (HCU) pump motor can cause a 'hard' brake pedal specifically during ABS activation, as it struggles to regulate fluid pressure effectively.
- The HCU pump motor rapidly cycles fluid during ABS events.
- It maintains pressure and returns fluid to the master cylinder.
- Failure affects only ABS-assisted braking.
- Conventional braking remains normal.
Memory trick: If ABS makes it hard, the pump motor's heart is failing.