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Hi guys! My brake pedal hisses, and once I lost boost for a couple seconds. I imagine the booster is about to fail, so I'm planning to change it soon. Is there anything I need to be careful of, rebuilt or new,best sources???
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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).
Regards,
Amarin.
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Thanks for the info Amarin! I just checked that out, the idle stays very smooth, but it does go up 100 rpm. It may indeed have a leaking diaphragm, but I wonder why it doesn't leak constantly when I press the brake? I'll try a new check valve, but I'm ready to order a booster if needed. My baby sister gave me a Cat 962841 LED worklight for Christmas, so I have the light thing covered. It's like having a pair of high beams in your hand, unbelievable!
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BTW, I just disconnected the master cylinder and pulled it away from the booster to check for leaks, I'm sure brake fluid would damage a new booster. Luckily, no leaks, dry as a bone. I still have not received the check valve I ordered on the 12th, and the store was fairly close, so I don't think I'll be ordering parts from Amazon again! I'll stick to Ebay or local suppliers.
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Check valve just came in, no difference, as I figured. NEW boost unit, $127.47 delivered, will be here Friday. Ebay rocks!!
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Got the new unit in place, but the Clymer manual neglected to mention adjusting the clevis rod. Braked were too tight, so I had to loosen the unit, shorten the clevis and adjust the brake light switch, so now they are to my liking. I should have known to do that, but I haven't ever had to change one before. When I first bought the 960 wagon I liked the power brakes because they weren't as highly boosted as the brakes on my work van. Well, the old booster was definitely on it's last legs, because the new one has plenty of boost! Thanks the help, gentlemen!!!
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Thanks for the update!
Well for a heavy car like 960 strong boost does help in braking confidently!
Regards,
Amarin.
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Amarin, I've gotten used to it, but the Taiwanese made unit definitely has a lot of boost. I hope the ABS is working well!
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Dear Mr. Weekend,
Hope you're well. It's possible that the brake booster's check valve has failed. This valve is a round black (or white) plastic unit (Volvo Part #270596, if your 960 is a '95 model) mounted in a black rubber grommet at the "ten o'clock" position.
This check valve - Bosch part No. 0204120018 - can be found at: https://www.amazon.com/Bosch-Power-Brake-Booster-0204120018/dp/B00757N5BC .
The booster units seem to be pretty durable. As the check valve is relatively cheap, accessible, and easy to install, it might be worthwhile to change it, to see if that helps matters.
Hope this helps.
Yours faithfully,
Spook
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Thanks, Spook, I'm well, hope you are. I ordered a check valve from Amazon, great price. It's worth a shot!
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