Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is pressure bleeding a brake system. The pressure bleeder is set to 15 PSI. After connecting the bleeder to the master cylinder reservoir and opening the bleeder screw at the furthest wheel, the technician notices that fluid flow is slow and inconsistent. What is the MOST likely cause?
- AThere is a significant air pocket trapped in the brake line.
- BThe master cylinder reservoir cap is not sealing properly.
- CThe bleeder screw is not fully open.
- DThe pressure bleeder's air supply is too low.
Show answer & explanationAnswer & explanation
Correct answer: B. The master cylinder reservoir cap is not sealing properly.
Pressure bleeding relies on a sealed master cylinder reservoir to force fluid through the system. If the reservoir cap is not sealing properly, the applied pressure will escape, leading to insufficient pressure to push fluid and air out effectively, resulting in slow and inconsistent flow.
Why the other options are wrong
- A. A significant air pocket would lead to a spongy pedal, but with proper pressure applied, fluid should still flow, albeit with bubbles. The symptom here is overall *slow and inconsistent flow*.
- C. While a partially open bleeder screw could restrict flow, the primary issue with a pressure bleeder is maintaining pressure in the reservoir. A not fully sealed cap is more fundamental.
- D. The pressure bleeder is *set* to 15 PSI, implying the air supply is adequate to maintain that pressure. If the supply were too low, the pressure wouldn't reach 15 PSI consistently.
Pressure Bleeding Requirements
The conditions and setup necessary for effective brake bleeding using a pressure bleeder, focusing on maintaining a sealed system.
- Requires a sealed master cylinder reservoir.
- Maintains constant pressure on the brake fluid.
- Allows one-person bleeding without pumping the pedal.
Memory trick: No seal, no pressure, no flow.