Rick's Free Auto Repair Advice

Hissing sound from brakes

What causes a hissing sound from under the dash

If you notice a hissing sounds from brakes while inside the passenger cabin, that’s the sign of 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.

When the engine is running and you’re not applying the brakes, engine vacuum is applied equally to both side of the diaphragm, so you get no braking assist. As you apply the brakes, a valve opens, allowing atmospheric pressure to flow in to one side of the diaphragm. Engine vacuum then pulls the diaphragm towards the master cylinder, forcing the push rod into the master cylinder.
cutaway view of vacuum boostercutaway view of brake vacuum booster

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

Posted on by Rick Muscoplat


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