Soft Brake Pedal Diagnostics

There is not a Diagnostic Trouble Code (DTC) for all the possible things that can go wrong with a vehicle. It also turns out that some physical symptoms within a brake system might lead you astray and replace master cylinders unnecessarily. This is where some active tests with your compatible diagnostic tool might lead you to the correct answers. This video will give you an overview of how to test the hydraulic section of an ABS unit and save some time and money in diagnosing a soft brake pedal.

Brake systems on cars have evolved to become safer with the addition of electronic controls such as Anti-Lock Braking Systems (ABS) and Electronic Stability Control (ESC) but it can make diagnosing soft or sinking brake pedals more difficult. There are various other reasons for a brake pedal to feel soft.

If an outlet solenoid valve in the hydraulic section of the module is stuck open, this is where a spongy or low pedal will be felt. As the master cylinder applies force to the brakes on that circuit and the outlet valve is not sealing, hydraulic pressure from the master cylinder will act upon the accumulator within the ABS Hydraulic unit. As the accumulator has a spring or a gas acting on the other side of the piston, the hydraulic pressure will compress it and give a soft, low or spongy pedal feel.

For more details on the diagnostic steps, see the October 2022 issue of VACC's TechTalk magazine.

VACC MotorTech members have access to an extensive technical information resource, a Technical Advisory Service and library staffed by experienced technicians, along with a comprehensive online resource, Tech Online, Tech Times Guide, Tech Estimate and Tech Talk magazine, plus online resources from Haynes - Haynes Pro and Haynes AllAccess, allowing you to repair and service more makes and models than your competition.

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