Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a brake inspection on an older vehicle equipped with a combination valve. The front brakes lock up prematurely during a hard stop, while the rear wheels barely engage. Which of the following is the MOST likely cause?
- AA seized proportioning valve
- BA faulty metering valve
- CA collapsed front flexible brake hose
- DAir in the rear brake lines
Show answer & explanationAnswer & explanation
Correct answer: A. A seized proportioning valve
The proportioning valve reduces hydraulic pressure to the rear brakes to prevent premature lock-up during hard braking. If it seizes or is stuck open, too much pressure will be sent to the rear, causing them to lock up. If it's stuck closed, too little pressure will go to the rear, leading to premature front wheel lock-up.
Why the other options are wrong
- B. A faulty metering valve (part of the combination valve) delays front brake engagement until rear shoes contact drums. If it were faulty and restricting front pressure, the front brakes would not lock up prematurely.
- C. A collapsed front flexible brake hose would restrict fluid flow to the front caliper, causing reduced front braking and potentially a pull, not premature front lock-up.
- D. Air in the rear brake lines would cause a soft or spongy pedal for the rear brakes, reducing their effectiveness, but would not directly cause the fronts to lock up prematurely.
Proportioning Valve
A hydraulic valve that reduces the fluid pressure to the rear brakes, typically during heavy braking, to prevent premature rear wheel lock-up.
- Prevents rear wheel lock-up.
- Located in the combination valve or inline.
- Failure can cause front lock-up or rear lock-up.
Memory trick: Balance the pressure, prevent the skid.