Brake Questions

[9fourteen]

I am the new owner of a 83 928S. Just brought it home this afternoon. The car runs pretty good, needs some cosmetic and interior work but the brakes are bad. They work but not very good. I figure the first thing I want to tackle is the brakes as that is a safety concern before I put this car on the road or do anything else. When I test drove the car the brake peddle felt a little spongy but not that bad. You just had to push really hard and you got pretty much no stopping power but it would come to stop eventually. You just had to think ahead. The PO said if he bled the brakes the peddle would be firm for a few weeks and the brakes would be good but after a month or so they would start having problems again. After a month he would have to bleed them again. He said the car needed a new brake booster and master cylinder. Before I go through the cost and time of replacing the booster and the MC, does that sound like the most likely cause? I don't see any leaks although I haven't really gone through the car very much yet. Are there any tests that can be done to determine the condition of the of booster and MC? Could it be something with the calipers? I haven't pulled the wheels and looked at the pads or rotors yet. Any suggestions on where to start would be good.

Thanks,
Don

Here is a pictures since everybody loves a picture.

[Danglerb]

Not much info, but that sounds like issues with the vacuum booster, either no vacuum or its bad.

928 brakes are progressive, not twitchy like American style power brakes. Push a little, stop a little, push a lot, stop a lot, unless the vacuum booster isn't working.

[Danglerb]

Note the not much info I say above, more and specific details on how it isn't doing what you want. How the peddle feels, hard or spongy etc.

[Mrmerlin]

so order these parts
a new ATE brake booster
an ATE master cylinder
a new ATE super gold brake fluid 1 liter
4 new stainless steel flex lines to replace the lines at each wheel
a new brake booster check valve
and small blue check valve
and 1 4 way vacuum splitter
1 tube of Dow corning 111 use this on the new grommets of the MC this will prevent water from corroding the seating areas remove the grommets and smear it on
a full set of EBC Redstuff pads

all in about 800.00 and you should be good to go

[Danglerb]

Or good to stop.

OTOH local place rebuilt my booster for I think $100.

[9fourteen]

Originally Posted by Mrmerlin

so order these parts
a new ATE brake booster
an ATE master cylinder
a new ATE super gold brake fluid 1 liter
4 new stainless steel flex lines to replace the lines at each wheel
a new brake booster check valve
and small blue check valve
and 1 4 way vacuum splitter
1 tube of Dow corning 111 use this on the new grommets of the MC this will prevent water from corroding the seating areas remove the grommets and smear it on
a full set of EBC Redstuff pads

all in about 800.00 and you should be good to go

I agree. That would fix it but you are essentially replacing the entire brake system. My question is more of how can I test the existing components to determine if they are bad or not and need replacing. The brake booster and the cylinder are the bulk of the cost. Some of those items might be ok and/or could just be rebuilt as Danglerb suggested. But, for $800 I would have a completely new brake system and confidence. I will probably go that route as it is not that much money and guaranteed to address the issue and hopefully stay fixed for several years.

[9fourteen]

Originally Posted by Danglerb

Note the not much info I say above, more and specific details on how it isn't doing what you want. How the peddle feels, hard or spongy etc.

As far as what it is doing or not doing, when you apply the brakes the car starts to slow down but takes a while. Breaking distance is much greater than it should be and if you had to make sudden stop, forget about it. If I were to guess I would say there is not enough pressure at the calipers to clamp down. Since the PO said he bled the brakes and it didn't really fix the problem I assume it is a mechanical failure versus air or moisture in the lines. That is assuming he bled them properly but they guy sounded competent. As far as the peddle, it didn't feel too bad. A little spongy but it is hard to gauge as all cars have a different feel and I am new to the 928 and when you don't get much of a result when you press the peddle you tend to push harder as you anticipate different results and that throws off the feel. The peddle definitely doesn't drop to the floor or anything like that. It had firmness there but not as stiff as I would like.

[5l32v4c]

I would go around the car first and see if you have any leaks, fixing any that you find. I would also to a thorough bleed yourself. I have used both the vacuum pump and the two-man method and found the latter to be the best way as the (cheapo) vacuum pump tended to draw air in around the bleed nipple.

If the pedal is spongey, I would have thought that indicates air is in the fluid. If it's the booster, I don't understand why it would get progressively worse - would have thought it would be bad all the time.

Change your pads and inspect / clean your rotors.

[neil30076]

Checking the booster is easy, put your foot on the brake pedal and apply pressure, maintain pressure - start the car, does the pedal go down, indicating the booster is at least helping you. Not a conclusive test, but it doesn't cost a cent!

[Mrmerlin]

FWIW I have replaced quite a few boosters since they are getting pretty old.
its is becoming more of a comon failure point.

based on the info from the OP.
testing has already been done and the parts list I posted is how you should proceed.
This will provide the best return for your money and in a week you forget how much was spent when you have a great set of brakes

[curtisr]

Originally Posted by 9fourteen

My question is more of how can I test the existing components to determine if they are bad or not and need replacing.

Ask this guy.

https://www.facebook.com/motodastrada.it/videos/10153109396431743/

[9fourteen]

I just bought a new booster, master cylinder rebuild kit, some grommets and fluid. I'll give that a go and see if that takes care of the problem.

[9fourteen]

Originally Posted by neil30076

Checking the booster is easy, put your foot on the brake pedal and apply pressure, maintain pressure - start the car, does the pedal go down, indicating the booster is at least helping you. Not a conclusive test, but it doesn't cost a cent!

I did this test, when i started the car the pedal went down but kept going down. Not sure if that is expected but it almost went to the floor as I kept pressure on it.

[honerboys]

sounds like the master