Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is evaluating a vehicle with a spongy brake pedal and excessive pedal travel. There are no external leaks, and the master cylinder reservoir is full. The technician suspects air in the system, but traditional bleeding methods (manual and pressure) have failed to completely resolve the issue. The vehicle is equipped with an Anti-lock Braking System (ABS). What specific procedure might be required to remove trapped air?
- AUtilizing a scan tool to cycle the ABS Hydraulic Control Unit (HCU) solenoids.
- BBench bleeding the master cylinder.
- CPerforming a vacuum bleed on each caliper.
- DReplacing all flexible brake hoses.
Show answer & explanationAnswer & explanation
Correct answer: A. Utilizing a scan tool to cycle the ABS Hydraulic Control Unit (HCU) solenoids.
Air can become trapped within the ABS Hydraulic Control Unit (HCU) solenoids or pump motor, especially after a component replacement or significant fluid loss. Standard bleeding methods often cannot clear this air. A scan tool is required to command the ABS module to cycle the HCU solenoids and/or pump, allowing trapped air to be pushed out into the brake lines where it can then be bled conventionally.
Why the other options are wrong
- B. Bench bleeding is done on a new master cylinder before installation; it won't remove air trapped elsewhere in the system.
- C. Vacuum bleeding is a method of bleeding, but if air is trapped within the HCU, vacuum bleeding at the calipers may not be sufficient to draw it out.
- D. Replacing hoses is a solution for hose issues (like swelling or leaks), not specifically for trapped air that resists normal bleeding.
ABS HCU Bleeding
A specialized brake bleeding procedure for ABS-equipped vehicles, often requiring a scan tool to activate the ABS pump and cycle solenoids to purge air trapped within the HCU.
- Required when traditional bleeding fails.
- Uses a scan tool to command HCU operation.
- Purges air from internal ABS valves/pump.
Memory trick: ABS air, scan tool's lair, purges with care.