Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairEasy
A technician is performing a bench bleed on a new master cylinder before installation. After connecting the lines and filling the reservoir, they slowly push the primary piston in and then release it, repeating the process. What is the PRIMARY purpose of this procedure?
- ATo ensure the master cylinder's internal seals are properly seated.
- BTo verify the master cylinder's internal pressure relief valve is functioning.
- CTo lubricate the primary and secondary pistons with brake fluid.
- DTo remove air from the master cylinder's internal chambers before installation.
Show answer & explanationAnswer & explanation
Correct answer: D. To remove air from the master cylinder's internal chambers before installation.
The primary purpose of bench bleeding a master cylinder is to remove any air trapped within its internal chambers before it is installed in the vehicle. Air in the master cylinder will lead to a spongy pedal and ineffective braking if not removed. Pushing the pistons slowly forces fluid through the internal circuits, expelling air bubbles.
Why the other options are wrong
- A. Properly seating seals is a byproduct of assembly, not the direct purpose of the bleeding action.
- B. While important, testing the pressure relief valve is not the primary goal of a bench bleed.
- C. Lubrication occurs as a natural part of filling with fluid, but it's not the primary *purpose* of the bleeding action itself.
Master Cylinder Bench Bleed
The process of pre-bleeding a new master cylinder on a workbench to remove internal air before installing it in the vehicle.
- Prevents a 'spongy' pedal after installation.
- Involves cycling the pistons slowly to expel air.
- Uses reservoir fluid to fill internal chambers.
Memory trick: Bench bleed 'cleans' the air from the master's guts.