Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A vehicle's brake system is equipped with a pressure differential valve. If a technician observes the brake warning light illuminated on the dashboard, but there are no visible external leaks, what is the MOST likely cause?
- AAn internal leak in one of the master cylinder's circuits.
- BLow brake fluid level in the reservoir.
- CA seized master cylinder piston.
- DA faulty pressure differential valve switch.
Show answer & explanationAnswer & explanation
Correct answer: A. An internal leak in one of the master cylinder's circuits.
The pressure differential valve is designed to detect a significant difference in pressure between the primary and secondary hydraulic circuits. This usually occurs when there's a pressure loss in one circuit. An internal master cylinder leak, even without external signs, would cause one circuit to lose pressure, triggering the differential valve and illuminating the warning light.
Why the other options are wrong
- B. Low brake fluid level typically illuminates a separate float-switch warning light in the reservoir, not directly the pressure differential valve light (though a low level could lead to air and a pressure loss).
- C. A seized master cylinder piston would likely cause a hard pedal or no pedal, but not necessarily trigger the differential valve light without a pressure imbalance between circuits.
- D. A faulty pressure differential valve switch could cause a false warning light, but if the question implies a *reason* for activation, an internal leak is a more direct and common hydraulic cause.
Pressure Differential Valve Operation
A safety valve that monitors pressure balance between the primary and secondary brake circuits and illuminates a warning light if a significant pressure difference is detected.
- Indicates a hydraulic pressure loss in one circuit.
- Does not stop the leak, only warns the driver.
- Often integrated into a combination valve.
Memory trick: Warning light: Low fluid, parking brake, or pressure imbalance.