A technician is performing a pressure differential test on a dual-circuit master cylinder. After applying the brake pedal, the technician notices that the pressure in the primary circuit is 800 PSI, while the pressure in the secondary circuit is 600 PSI. What does this pressure difference MOST likely indicate?
- AAn internal leak within the master cylinder.
- BA properly functioning master cylinder and brake system.
- CExcessive clearance or a leak in the secondary circuit.
- DA restriction in the primary circuit's brake line.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive clearance or a leak in the secondary circuit.
In a properly functioning dual-circuit master cylinder, the primary and secondary circuits should produce nearly equal hydraulic pressure when the pedal is applied. A significantly lower pressure in one circuit, especially the secondary circuit, suggests a problem in that circuit that is preventing it from building or holding pressure effectively. This could be due to excessive clearance in the wheel cylinders/calipers, air in the lines, or a leak (internal or external) within that circuit.
Why the other options are wrong
- A. An internal master cylinder leak would typically cause both circuits to lose pressure or the pedal to sink, but a differential like this points to an issue *within* one circuit's ability to build pressure.
- B. A 200 PSI difference indicates a problem, not proper function.
- D. A restriction in the primary circuit would likely lead to *higher* pressure in the primary circuit (as fluid is blocked) and potentially lower or no pressure in the secondary if it's downstream, but the primary pressure itself would be affected differently.
Master Cylinder Pressure Differential
The comparison of hydraulic pressure between the primary and secondary circuits of a dual master cylinder, used to diagnose system imbalances.
- Pressures should be nearly equal in a healthy system.
- Significant differences indicate issues like leaks, air, or component wear in the lower-pressure circuit.
- Typically measured with pressure gauges at the wheel cylinders/calipers.
Memory trick: Pressure balance tells the tale, circuits should be equally hale.