December 6, 2023

This Tech Tip article will cover brake hydraulics, including the loss of pedal pressure. A customer comes into the shop and complains of losing pressure when they press the brake pedal. They’re pushing the pedal to the floor, but the amount of stopping power they receive isn’t equal to the effort they’re putting into that pedal. Have you seen this one before?

It can be a scary feeling for your customer and should indicate to you that there may be something wrong with one or some of the hydraulic components within the braking system.

So where should you start when you have this problem on your hands? One of the most commonly under or misdiagnosed causes of pedal pressure issue is the quality of the piston seal between the brake master cylinder and the brake booster.

If the seal is compromised, brake fluid can leak from the hydraulic system into the atmosphere, or it can leak internally around the piston seals. Loss of pedal pressure can also be caused by a worn rear seal in the master cylinder, which allows air to enter the cylinder as the brake pedal is released. That air can sometimes be pumped back into the hydraulic system, causing a spongy pedal feel.

Nowadays, new vehicles are equipped with ABS, or anti-lock brake system, which controls brake pressure across the four wheels, meaning the brake fluid in the vehicle is also controlled electronically. The electronic brake force distribution system, or EBD, controls brake fluid being pumped into the brake lines as brake force is applied to the system hydraulically through the brake pedal. If there is an issue with the ABS or EBD, this could potentially cause a lack of pedal pressure.

A vehicle’s hydraulic system can be complex, and the plot thickens on newer vehicles where electronic control of fluids is added into the mix. So using your scan tool to run a full diagnostic report can ultimately be the best way to tell if your customer’s pedal pressure issue is electronic or hydraulic.

Even though the majority of today’s vehicle hydraulic systems are electronically controlled, you must still know how to service and repair these parts in the event of a front-end collision, especially on older models.

Stay up to date with manufacturer installation requirements and specifications so you’re equipped with the necessary knowledge to perform these repairs correctly.

Regardless of whether a vehicle is equipped with ABS, you should always check the brake master cylinder when a customer complains of lack of pedal pressure. This way, you can identify any cracks, leaks, or seal issues right away and replace parts accordingly.