Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairEasy
A technician is diagnosing a spongy brake pedal on a vehicle. After inspecting for external leaks and ensuring proper fluid level, they attempt to bleed the brakes manually. During bleeding, the technician observes that the fluid coming from the bleeder screw contains numerous small bubbles, even after several cycles. Which of the following is the MOST likely cause of this condition?
- AA faulty proportioning valve.
- BAir entering the system through the bleeder screw threads.
- CA collapsed flexible brake hose.
- DAn internal leak in the master cylinder.
Show answer & explanationAnswer & explanation
Correct answer: B. Air entering the system through the bleeder screw threads.
Air entering through loose bleeder screw threads during manual bleeding is a common cause of persistent bubbles, leading to a spongy pedal. This is often mistaken for air from within the system.
Why the other options are wrong
- A. A faulty proportioning valve affects pressure distribution to the rear brakes but would not typically cause continuous bubbles during bleeding.
- C. A collapsed hose would typically restrict fluid flow, leading to a hard pedal or no fluid, not persistent bubbles during bleeding.
- D. An internal master cylinder leak causes the pedal to slowly sink, not persistent bubbles during bleeding, unless it's drawing air from the reservoir which is less likely to show as continuous small bubbles at the caliper.
Bleeder Screw Air Intrusion
During manual brake bleeding, if the bleeder screw is not sufficiently tightened, air can be drawn into the brake caliper or wheel cylinder around the threads when the brake pedal is released, giving a false indication of air still being present in the hydraulic system.
- Occurs during manual bleeding.
- Bleeder screw not tight enough.
- Draws air in on pedal release.
- Produces persistent small bubbles.
Memory trick: Bleeder screw's loose, air's a goose!