RWD - My 960's brake pedal hisses sometimes when I step on it....

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My 960's brake pedal hisses sometimes when I step on it.... 900

In my experience a hiss when the brake pedal is pressed means the brake booster's diaphragm is leaking. I'm referring to the big 8" rubber diaphragm within the booster (thats the size for my 940). My previous booster did that last time and idle became erratic for a while. If fact almost every time I pressed the pedal the idle became erratic. This how I came to conclude about the leaking diaphragm. Try doing that on your car at idle.

The big diaphragm separates the vacuum side (the one facing brake MC) and the atmospheric side (cabin side). When the brake pedal is pressed an atmospheric valve opens at cabin side allowing more air to go in thus allows more atmospheric pressure acting on the diaphragm which then acts on the push-rod (the vacuum side is already made vacuum by the engine suction). The push-rod then pushes the MC. This is how assisted/power braking is accomplished. There's a sponge filter at the atmospheric valve to lessen the suction noise BUT that extra air that goes in at atmospheric side DOESN'T go into vacuum side (separated by the diaphragm). If they mix then the diaphragm is leaking/torn/damaged. That erratic idle is caused by THAT extra air reaching the air/fuel mixture when pedal is pressed.

In short you might have to replace the booster. Replacement is not difficult but its time consuming plus you do have to lie on the driver's footwell to undo the nuts and pedal pins. A good lighting is needed here. You'd need to undo the two nuts at the MC side and push the MC gently out of the way. No need to dismantle any brake lines. The gasket between booster and firewall is important as to prevent engine noise from going into cabin. If the gasket is flattened, a DIY gasket from gasket paper is suffice (with some RTV). I neglected this gasket and had to repeat the entire job again.

Replacement diaphragm may be obtained from eBay and some companies on the web but DIY booster rebuilding needs special tools and skills to split open the booster case (which is beyond the scope of consumers including myself).



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New 1 My 960's brake pedal hisses sometimes when I step on it.... [900]
posted by  Mr. Weekend  on Sat Feb 11 12:57 CST 2017 >

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