m21sniper

My 83 has been acting in a similarly goofy fashion lately(almost the same symptoms as the dog has).

I'm going to bleed them first to see what happens.

FlyingDog

I messed with the brakes some more today. The car hasn't been started since wednesday, but the booster still had vacuum. Both check valves appear to be working properly. I rolled up a paper towel like a rope and stuffed it in the booster. It came out clean, so the master isn't leaking.

Without vacuum, the brakes are hard, but not quite solid. The pedal doesn't drop when holding pressure. When I start the car, the pedal gets very soft. While waiting for the engine to warm up, pressing the brake pedal slightly effects idle. All of this makes me think the booster is bad, but that doesn't make any sense. If the booster is bad, the pedal should be hard not soft.

How does ABS fail? I removed the fuse from the ABS relay and there was no difference. The fuse looks good. The relay appears to work when applying 12V across 31 and 15 to operate the coil.

mulik51

I don't know if this helps, but on my ford, i have a similar brake problem. Do you have a squshing sound when you don't press the brake? Becouse I diagnosed my ford and it was old brake fluid compressing. I would try to rebleed the brakes before changeing anything expensive...

Klim

FlyingDog

What do you mean by squishing when I don't depress the brake? It squishes pressing and releasing the brake pedal, but I haven't noticed a sound any other time.

I just noticed this in an old post by glenL:
"There's a big Y up by the plenum. It's acutally a vacuum boster. The base of the Y goes to the plenum. The straight-ahead leg goes to the brakes. The other arm runs down to the lower part of the manifold."
I think mine is reversed. I don't know if that makes a difference.

edit: mine is correct according to the WSM.

AO

Well it sounds like it's either the master or the booster. If I had to put money on it I'd say the master is bad - I know you just replaced it. But based on your description, when the car is on, the pedal goes soft - this tells me that the booster is doing its job - multiplying your brake force. So where else could you loose pressure? If you're not leaking fluid, I would think then either you have a ton of air in the system or the selas in the master are bad. Since you've bled the system a bazillion times, I'd say the master seals are bad. Sorry, but this wouldn't be the first time a bad part was installed.

Give Rixter a call, I'll bet he can get you a know good MC from one of his wrecks cheaply to confirm this is the issue. Then I'd look at reverse bleeding the system just to be sure there's no air in the system.

Good luck!

mulik51

I have a squshing sound(like shhhhhhhh) all the time. It gets worse when I press the pedal, but if I press it very hard, there is no sound at all. Does it sound familiar?

Klim

FlyingDog

I guess I'll call Jim B. about getting my master swapped. At least it's still under warranty for another couple days.

Mine just makes noise when I'm moving the pedal up or down. If the pedal is all the way up or is being held in place I haven't noticed any noise.

FlyingDog

It's 31 degrees, snowing and I just bled my brakes on the side of the road in front of my house. There was a little air in the rear lines, but not much. The pedal was a little firmer with the engine off, but as soon as I started it, the pedal became extremely soft again.

I called Jim. He said if it's not leaking and the pedal is firm without the power assist, the master is good. That's what I thought. It looks like it's the booster.

Thanks for the help guys.

AO

SHould have posted this earlier.. "WWJBS?"

I guess we know now. Let us know how it goes.

BTW, 31*, snowing, on the side of the road! Kudos to you!

dr bob

The Big Factor that nobody has mentioned is brake pads. Related to that is the calipers. Replace the pads with some known good type. OEM's are fine, stay away from race-type pads though. Old pads may get soft enough to cause a problem. Pads are relatively cheap as 928 parts go, so don't be afraid. Pushing the pads back in the caliper 'preloads' the seal some, as part of the pad replacement process. As the new pads bed in, the caliper seals will find a normal position that just pulls the pads back just enough to clear the rotors.

One other Big Factor is the amount of travel needed at the pads to get the pads to contact the rotor. If you have a slightly warped rotor, you might find that you have a pusing pedal. If the front wheel bearings are abit loose (and it doesn't take much...) the wobbling rotor will push the pads back a little further.

Flushing the brakes is a challenge, since debris settles to the low points in the calipers, and the bleed valves are at the top to allow air to float to the top and escape. You can flush with the hose disconnected, rotate the calipe so that the hole is at the bottom with the bleed valve removed, and finally bleed with the caliper installed in the normal position. S4+ calipers can be flushed with the cross-pipe removed I guess, but I've never tried that.

It often takes a lot of flushing and bleeding to get the junk and all the air out of a brake system that hasn't been maintained. Rust and other unidentifiable crud seem to accumulate in cars that infrequently enjoy a fluid flush.

So-- Start off with the basics of a good brake pad replacement, wheel bearing adjustment, and a fluid flush and bleed. Go after other hydraulic problems if they exist. For instance, the chance of getting a new bad master cylinder are vey small. More likely is that there is another problem, especially considering that the problem hasn't changed with the new MC. The rest of the system is just hoses, calipers, pads. Oh, and the fluid.

FlyingDog

Mini update: I bought a new booster off ebay (how could I not for $113?), but it appears that I may not need it. I pulled the ABS relay when I was doing the testing 2 weeks ago. The brakes have improved drastically since the ABS relay has been pulled. I haven't put it back in yet, so I don't know if the ABS was preventing proper bleeding, is bad, or it's just a coincidence. I haven't gone on a real road test because of the throwout bearing going, but I drive it around a couple blocks when I have to move it back and forth across the street for parking rules. I can actually lock the wheels up sometimes and the pedal isn't nearly as soft now.

AO

Matt-
I'm curious what the final outcome was on this? I noticed my pedal has bee quite soft lately. I'm going to re-bleed to see if that helps, but like you, I cannot get the ABS to engage. It's definately not vacuum, but thought it could be the master. But after reading your last post, I'm thinking I should pull the ABS relay and give it a whirl, but wanted to hear from your recent experience first.

FlyingDog

I haven't fixed it. The car is in the garage with a bunch of electrical parts disassembled. I have a new brake booster to try, but it'll be a while before I know if that fixes it. Sorry I can't help you more.

dfroelicher

I have a possible related brake issue.

The brake pressure light on the instrument cluster has been flashing for nearly a year now and the brakes are just fine everyonce in a while the fluid level light comes on and I just add more fluid.

Any ideas or thoughts? Could it be related to the booster or master or neither?

AO

Matt-
What good are you! Get that car back on the road and figure it out, would ya!

dfroelicher-
You must have a leak somewhere. No other way to explain the loss of fluid. If it's a manual, I'd look at the blue hose. That can easily leak at the resevoir. After that, check brake line, seals, etc.