Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairHard
A vehicle exhibits a soft or spongy brake pedal, and the technician suspects air in the hydraulic system. After bleeding the brakes at all four wheels, the pedal feel improves but is still not firm. Which of the following is an additional step that should be performed, especially on vehicles with ABS, to ensure complete air removal?
- AReplacing the master cylinder.
- BAdjusting the brake pedal free play.
- CRefilling the brake fluid reservoir with a different type of fluid.
- DBleeding the ABS hydraulic control unit (HCU) using a scan tool.
Show answer & explanationAnswer & explanation
Correct answer: D. Bleeding the ABS hydraulic control unit (HCU) using a scan tool.
Modern ABS systems have internal valves and accumulators within the hydraulic control unit (HCU) that can trap air. Conventional bleeding methods often cannot remove this trapped air. A scan tool is required to cycle the ABS solenoids and pump, allowing the trapped air to be pushed out and bled through the wheel bleed screws.
Why the other options are wrong
- A. Replacing the master cylinder is a last resort if bleeding and ABS cycling don't resolve the issue, and not a primary step for air removal.
- B. Adjusting brake pedal free play affects the initial pedal position, not the firmness or presence of air in the hydraulic system.
- C. Using a different type of brake fluid is incorrect and can damage the system; only the specified DOT fluid should be used, and refilling alone doesn't remove air.
ABS Bleeding
Modern ABS systems may require a scan tool to cycle the hydraulic control unit (HCU) solenoids and pump to purge trapped air that traditional bleeding methods cannot remove.
- Air can get trapped in ABS module valves.
- Scan tool activates ABS components.
- Necessary for complete air removal and firm pedal after system intrusion.
Memory trick: Spongy pedal, ABS light? Scan tool's the key to make it right!