Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairEasy
A vehicle with a dual-circuit master cylinder experiences a sudden, complete loss of braking in the front circuit. What is the primary function of the pressure differential valve in this scenario?
- ATo prevent wheel lock-up by modulating brake pressure.
- BTo maintain equal pressure to both the front and rear circuits.
- CTo proportion brake pressure between the front and rear axles.
- DTo alert the driver of a pressure imbalance between circuits.
Show answer & explanationAnswer & explanation
Correct answer: D. To alert the driver of a pressure imbalance between circuits.
The pressure differential valve (often part of a combination valve) detects a significant pressure imbalance between the two hydraulic circuits (due to a leak) and activates the brake warning light on the dashboard to alert the driver.
Why the other options are wrong
- A. Preventing wheel lock-up is the function of the ABS system and proportioning valve, not the pressure differential valve.
- B. Maintaining equal pressure is not its primary function in a failure scenario; it detects an *imbalance*.
- C. Proportioning brake pressure is the function of a proportioning valve, also often part of a combination valve, but distinct from the differential function.
Pressure Differential Valve
The pressure differential valve detects a significant pressure loss in one hydraulic circuit of a dual-circuit system and activates the brake warning light to alert the driver.
- Monitors pressure balance between circuits.
- Activates warning light upon imbalance.
- Does not directly control brake pressure or prevent lock-up.
Memory trick: Differential senses imbalance, then warns of danger's chance.