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What is wrong with my brakes?

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Old 11-22-2005, 10:26 PM
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FlyingDog
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Default What is wrong with my brakes?

When I bought the car, the brakes were squishy and would only hold if used moderately hard, otherwise the pedal would slowly drop. When using the brake pedal there is a swishing air sound, like a leak. I bled the brakes with Superblue and they improved greatly when working, but still had the same symptoms of not working. I bought a master from 928I last december and finally got around to installing it 2 or 3 months ago. It still has the same symptoms. There is no fluid in the booster or leaking from anywhere else.

Now for the strange part. While following Beerfish home from shaaark's two weeks ago the brakes magicly improved. Some idiot on a cell phone in a Nagivator cut between us while we were cruising at 70-80. When we came upon traffic the idiot wasn't paying attention and nearly rearended Beerfish. I couldn't see around the giant POS so I had to slam on my brakes when his/her tail went in the air. I heard a pop from the firewall and the pedal dropped. Being used to having to push the pedal twice to get good response I quickly hit the pedal again and the brakes worked. The pedal no longer sinks on its own. The brakes still feel soft and make a swishing sound from the booster. They still won't lock or kick in the ABS (which I have never had kick in...).

I've done the booster tests listed online (Rennlist and Google cache of 928I catalog). Master is new. Soft lines looked new when I bought it. Fluid is new and well-bled. Pads have plenty left and I don't see anything wrong with the rotors.

Is it:
Booster (normal mode of failure would be hard pedal)
Something else vacuum related (how?)
Master (if so I have to return my bad one in the next week or so to Jim B)
ABS failure (trying to do it's anti-lock thing all the time, or leaking internally)
Gremlins
Old 11-22-2005, 11:01 PM
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ErnestSw
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Slave?
Old 11-22-2005, 11:05 PM
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Do you mean all 4 calipers? Nothing is leaking. Although, when I first bled them the right rear took a pump of the pedal to start flowing even with 10psi in the power bleeder. It felt like a clog broke loose. That was also the hardest bleeder to open.
Old 11-22-2005, 11:57 PM
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G Man
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Matt. I have a similar problem with the brakes on my 81'. I really have to stand on them to stop. I am pretty sure my problem is the booster. I have done the test where you drain all the vacume out of the system and press the brake pedal and start the car. The pedal does go down but I still think the booster is the problem. I have been unable to get the wheels to lock up no matter how hard I stomp on the pedal.

Last edited by G Man; 11-23-2005 at 12:51 AM.
Old 11-23-2005, 12:01 AM
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FlyingDog
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That's what I think also, but knowing how a booster works I can't figure out what would be wrong causing those symptoms. You 81 obviously doesn't have ABS.
Old 11-23-2005, 12:14 AM
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Randy V
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I think Ernest was referring to the 'slave' cylinder. The manual transmission cars, such as yours, utilize a slave cylinder (a slave to the brake system) to actuate the clutch.

A failure of the slave clutch system can result in adverse performance of the braking system, and vice versa.

That may also explain your signature tag line.
Old 11-23-2005, 12:18 AM
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I figured that's what he was refering to, but gave him the benefit of the doubt. The slave wouldn't effect brake pressure as the two are linked on the low pressure side of the two masters and neither system leaks.

Clutch hydraulics are perfect. It's the throwout bearing that's bad.
Old 11-23-2005, 09:09 AM
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AO
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The "swooshing" sound immediately makes me think it's your booster. Remember the booster is there to amplify your actions and increase the brake force. Have you ever tried to stop an old car with the engine off? You really need to stand on the pedal becasue there's no vacuum to actuate the booster. In your case it sounds like the booster is working somewhat, but there's probably a tear or leak in the diaphram.

Don't know what the POP would have been. Maybe coincidental, or maybe the diaphram shifting in the booster? Not sure.
Old 11-23-2005, 11:44 AM
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That's what I don't understand. If the diaphragm is torn or leaking, the brakes should be solid like braking with the engine off. My booster seems to be working too much to assist the pedal without assisting the braking.
Old 11-23-2005, 01:18 PM
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GlenL
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Drive, bleed, repeat.

I've experienced this before. Once I'm certain the MC is screwed up, I'll bleed it again and it gets better. Eventually everything is fine. It seems that bubbles get stuck in the pipes, calipers or MC that are not flushed out.

Maybe there's some trick. Replacing lines seems to be much easier to get right than replacing the master.
Old 11-23-2005, 02:31 PM
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Peter F
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Recently replaced my brake tubes at all four wheels with steel braided.
After a good bleed the feel is simply awesome compared to before.
Stopping power is greatly improved, the old tubes were stamped -81....
So not so strange they where spongy, a change was well due.
Will do the same on my GT during winter

Cheers/Peter
Old 11-27-2005, 11:50 PM
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I did my bi-weekly side of the street changing tonight. While waiting for it to warm up, I played with the brake pedal. While sitting, the pedal requires almost no force to push it near the floor. Resistance will increase about 3/4 down but it will sink to the floor. There is a loud swishing noise whether pushing hard or soft. The pedal will not pump up.

Once I started driving around the block, the pedal is harder if I'm letting of the accelerator. Once it's back to idle, it's pretty soft again. The braking ability is similar to the force needed on the pedal. Hard pedal = working brakes. Soft pedal = very squishy ineffective brakes.

I'm beginning to think the 928I stainless lines might be worth trying, but I don't think they'll fix the problem. I still think it's vacuum and/ or booster related, but can't figure out how or why.
Old 11-28-2005, 12:09 AM
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Well that helps. Since the booster runs off of stored vacuum, and your brakes seem better at higher engine speeds - and worse at lower, I predict you have a vacuum leak somewhere. Check your check valve first. Then use a mighty vac to isolate any other leaks. Could still be the booster though ... But could be something like a bad hose . Could also be the vacuum pod itself in the driver fender behind the wheel. Let us know.
Old 11-28-2005, 10:50 AM
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Sounds like you've got air in there and maybe a weak (leaking) master.

Stainless lines won't fix that in themselves. As it'll require a lot of bleeding, the problem might be fixed in the process. That it's sometimes good and sometimes bad isn't related to rubber lines.

Look inside the booster for fluid.
Old 11-28-2005, 01:56 PM
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I had a similar problem a while back in my 79. It turned out to be the rubber seals in the MC reservoir going bad. Eventually, the seal let loose (while parked) and the reservoir drained out. Not sure about the newer ones, but I had the same brake feel that you are describing.


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