Troubleshooting the Hissing Sound When Pressing Brake Pedal

What causes a hissing sound when pressing the brake pedal?

If you notice a hissing sound when pressing the brake pedal, and the sound comes from under the dash, the problem is usually a failing vacuum booster. The vacuum booster uses engine vacuum to increase braking power. The booster uses a rubber diaphragm and a push rod. The rod is connected on one side to your brake pedal and the other side pushes the master cylinder piston.

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 the combustion process. This vacuum is then used to create a force differential across the diaphragm, which is connected to the brake pedal on one side and the master cylinder on the other.

When the driver presses the brake pedal, a valve opens, 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 hissing sounds from brakes while you’re inside the cabin. This is a fairly easy fix. Remove the two bolts holding the master cylinder in place and pull it away from the vacuum booster. Then disconnect the push rod from the brake pedal. Next, remove the four bolts or retaining nuts holding the vacuum booster onto the firewall.

Replace the vacuum booster with a rebuilt unit. Reverse the procedure to reinstall.

©, 2015 Rick Muscoplat