Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a bench bleed on a new master cylinder. After filling the reservoir and cycling the pistons, they notice small air bubbles continue to appear in the fluid being returned to the reservoir, even after numerous cycles. What is the MOST likely reason for this persistent aeration?
- AThe bleeder tubes are not fully submerged in the reservoir fluid.
- BThe brake fluid used is contaminated with water.
- CThe bleeder tubes are submerged too deeply in the reservoir fluid.
- DThe master cylinder is defective and drawing air internally.
Show answer & explanationAnswer & explanation
Correct answer: A. The bleeder tubes are not fully submerged in the reservoir fluid.
During a bench bleed, the bleeder tubes must be fully submerged in the brake fluid. If they are not, air can be drawn back into the master cylinder on the return stroke, leading to persistent aeration and preventing a complete bleed.
Why the other options are wrong
- B. Water contamination in brake fluid can lower the boiling point, but it typically doesn't cause persistent aeration during a bench bleed unless it's boiling, which is unlikely during this procedure.
- C. Submerging the tubes too deeply would not cause aeration; it would just ensure the air escapes without being drawn back in.
- D. While possible, a defective master cylinder drawing air internally is less likely than an incorrect bench bleeding technique when persistent aeration is observed with new fluid.
Master Cylinder Bench Bleed
The process of pre-bleeding a new master cylinder on a workbench to remove air from its internal passages before installation, ensuring proper function.
- Prevents air from entering the system upon installation.
- Uses short hoses to cycle fluid back into the reservoir.
- Hoses must remain submerged to avoid drawing in air.
Memory trick: Tubes must dive, to keep air alive (out).