Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is bleeding a hydraulic brake system. They notice that after repeated pedal presses, the fluid coming out of the bleeder screw still contains small, consistent air bubbles. The bleeder screw is tightened properly, and the fluid reservoir is kept full. What is the MOST likely reason for the persistent air bubbles?
- AAir being drawn in around the bleeder screw threads
- BA leak in the vacuum booster diaphragm
- CA partially seized caliper piston
- DA faulty master cylinder internal seal allowing air to enter
Show answer & explanationAnswer & explanation
Correct answer: A. Air being drawn in around the bleeder screw threads
Even if tightened, air can be drawn around the threads of a bleeder screw if they are not sealed, especially during the suction phase of pedal release, giving the appearance of persistent air from the system.
Why the other options are wrong
- B. A leak in the vacuum booster diaphragm would cause a hard pedal, not introduce air into the hydraulic fluid during bleeding.
- C. A partially seized caliper piston would cause dragging or uneven braking, but would not introduce air into the hydraulic system during bleeding.
- D. A faulty master cylinder internal seal would typically cause a sinking pedal, not persistent air bubbles from a bleeder screw if the external system is sealed.
Bleeder Screw Air Intrusion
Air can be drawn into the brake system around the threads of a bleeder screw during the bleeding process, even if the screw is tight, if the threads are not properly sealed.
- Mimics air coming from inside the system.
- Often resolved by applying thread sealant or ensuring proper bleeder screw condition.
- Can lead to a spongy pedal if not addressed.
Memory trick: Bleeder threads, not the fluid, might be letting air in through the back door, even when tight.