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Strange brake problem: pedal gets soft at lights

 
Old 03-09-2012, 04:29 PM
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kevinlieb
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Default Strange brake problem: pedal gets soft at lights

I am having a strange issue in my 88 S4 Auto. Its not really a "problem", maybe a quirkiness of the car or its age? I don't know.

When I get up to a traffic light and I'm waiting at the light, completely stopped, the brake pedal is firm. After a couple of seconds it gets slightly softer and the car will move forward if I leave the same constant pressure on the brake pedal. I feel the pedal go down a bit so I keep the pressure on and the car doesn't move.

Anyone notice this and know of a possible reason for this? I'm thinking there is a leak in the brake booster? Or the brakes need a good bleeding?

Perhaps I should also mention: the brakes work fine when driving. No problem stopping, no pulling, no issues. I do get a nasty squeal after a lot of stop and go traffic with moderate braking but I have new dampeners to fix this.
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Old 03-09-2012, 04:34 PM
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G Man
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time for a new master cyl
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Old 03-09-2012, 04:36 PM
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Speedtoys
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Also..check for brake fluid eatin the carped under your feed at it leaks back into the cabin...if its the MC
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Old 03-09-2012, 04:38 PM
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Andre Hedrick
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Do you hear any hiss/suction sounds?
Mine did the above but did not change the brake behavior.
However, since I just replaced mine and the brake rod through the booster is solid, I suspect your master cylinder is worn.

You can plug the line on to the booster and drive (carefully, as the pedal becomes a monster brick on the leg) and repeat your test, and if it happens w/out the BB enabled, you have a rebuild/replace master cylinder project this weekend.
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Old 03-09-2012, 04:39 PM
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Is your brake fluid in the Master Cylinder discolored? Most of it sold DOT 4 is clear. So if its muddy looking like coffee or cola, your seals are deteriorating and this will lead to eventual brake failure. I would not even try to rebuild it. I would get a new or reman -Master Cylinder.

If it was the brake booster you would have a hard pedal, but it would take more effort.
Slow to the floor is usually master cylinder rubber seals going bad.
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Old 03-09-2012, 04:56 PM
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i did notice the carpet peeling back in front of the gas/brake pedals. That makes me fear its the fluid leaking in. I'll take a look. Thanks for the advice all!
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Old 03-09-2012, 05:34 PM
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There is a seal in the back of the Master Cylinder that can leak also. No matter what, it needs to be replaced. (if the seals [internal or external] are going bad../)
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Old 03-09-2012, 05:50 PM
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I checked the carpet: its falling apart (backing coming off is causing the peeling, making a big mess) but no dampness of brake fluid. Fluid was clear as far as I can tell. There are a lot of old vac lines around there that probably need rehab but no obvious cracks. I'll take a more careful look later when work isn't getting in my way...
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Old 03-09-2012, 06:52 PM
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When you order your new MC don't forget to get two new rubber seals that join the MC to the resivour. Oh, and welcome to 928 world!
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Old 03-09-2012, 07:01 PM
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No idea if its related, but I've had the same thing happen in my 91 Mustang, pull up to a stop, and after a second or so brake pedal sinks a inch or so. I'm not sure what got replaced, but doesn't do it anymore.
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Old 03-09-2012, 07:27 PM
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danglerb: it would be nice to know what fixed the problem in that case.

The more I think about it the more I think I am in denial. What makes carpet fall apart faster than brake fluid? Nothing I can think of. I just have to check thru my receipts and see if the PO did this already, and the carpet damage is from the last time this failed. But there is an excellent chance this is the problem...
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Old 03-09-2012, 07:43 PM
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Brake fluid cannot leak from the rear of the master to the interior of the car. It will leak into the booster if it leaks from the rear. As stated a faulty booster will cause the pedal to be hard. Is the fluid low ? The pedal will drop with constant pressure if the master is leaking internally.
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Old 03-09-2012, 07:46 PM
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SeanR
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If he is losing vacuum he will get a soft pedal at idle.
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Old 03-09-2012, 07:52 PM
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Any loss of vacuum that would affect the boosters perfomance will cause the pedal to get harder. That is why power brake boosters have a vacuum check valve.
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Old 03-11-2012, 03:52 PM
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I've heard when the stock rubber brake lines start to weaken it will have the same effect, they basically balloon a little bit under extended pressure creating more volume in the brake system. Fact or fiction?
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