Troubleshooting the Hissing Sound When Pressing Brake Pedal

What causes a hissing sound when pressing the brake pedal?

A hissing sound when applying the brakes in a car can be concerning for any driver. This sound typically indicates an issue within the vacuum brake booster, which provides the power assist that gives you power brakes. I’ll explain a bit more about what the brake booster does and what makes it cause a hissing sound when pressing the brake pedal.

How a vacuum brake booster works

The vacuum brake booster consists of two main components: a vacuum chamber and a diaphragm. The vacuum chamber is connected to the engine’s intake manifold, where a partial vacuum is created during combustion. This vacuum is then used to create a force differential across the diaphragm, connected to the brake pedal on one side and the master cylinder on the other.

A valve opens when the driver presses the brake pedal,

This image shows the in-cabin portion of the vacuum brake booster. Notice the eyelid where the brake pedal arm connects and the protective boat that prevents dirt from getting onto the seal.

allowing atmospheric pressure to enter the vacuum chamber. This pressure differential between the vacuum chamber and the atmospheric pressure side causes the diaphragm to move, applying force to the master cylinder. The master cylinder, in turn, sends brake fluid through the lines to the individual brake calipers or drums, engaging the brakes and slowing the vehicle down.

However, if the atmospheric valve leaks air, you’ll notice a hissing sound when pressing the brake pedal. This is a fairly easy but not necessarily cheap fix. You can’t replace just the seal. Instead, you replace the entire vacuum booster

How to replace a leaking vacuum brake booster

The vacuum brake booster and master cylinder are mounted on the firewall

1) Purchase a rebuilt unit from an online seller or local auto parts store. They will charge you a core charge because they want the old unit returned so it can be rebuilt.
2) Pop the hood and locate the master cylinder. Remove the two bolts holding the master cylinder onto the vacuum booster
3) Gently pull the master cylinder away from the vacuum booster, taking care not to kink any of the steel brake lines.
4) Disconnect the push rod from the master cylinder or vacuum booster and set it aside, taking care to notice its orientation.
5) Disconnect the vacuum hose connected to the vacuum booster.
6) Crawl under the dash and disconnect the brake pedal arm pivot that connects to the vacuum booster. Remove the four bolts or retaining nuts holding the vacuum booster onto the firewall.
7) Install the new vacuum booster and tighten the bolts to spec.
8) Install the push rod into the new booster
9) Install the master cylinder, properly aligning the O-ring with the master cylinder and booster (if equipped with an O-ring).
10) Install the master cylinder bolts to the vacuum booster with a rebuilt unit. Reverse the procedure to reinstall.

©, 2015 Rick Muscoplat