drscottsmith

All - '83 928s

Trying to narrow down my brake issues and want to summarize my findings...

1) MC looks like it went through WW2 with the rust on it. Was empty when I got car. Filled it with fluid to see what was going on - no visible fluid leaks anywhere under car. All symptoms below were the same before and after adding fluid.

2) Rear brakes will not bleed. Well, drivers side will push fluid into my bleed line about 2". Pass. side - nothing. Have not even tried the fronts.

3) I have to stand on the brake pedal to get any stopping. I can hear the shoes grabbing the discs when I do this as the car stops.

4) If I step on the brakes and start the car, I can feel the pedal depress slightly, and when I turn car off it firms back up - leads me to believe the booster is ok.

5) I can hear a fairly pronounced vacuum hiss in the cabin anytime I press the brake pedal.

6) Clutch seems to be working fine. I am attributing difficult shifts into 2 and 3 to worn syncros and the need for new gearbox oil (coming Saturday).

Any thoughts based on symptoms? I am thinking a piston in the MC is frozen. I feel like the booster is ok - I know I likely have some other vacuum issues based on other symptoms, but am getting ready for a top end refresh and will change all lines anyway. It does appear to be getting and holding at least some vacuum though.

Thanks as always -

-scott

dr bob

Empty fluid reservoir means air and moisture exposure for the MC internals. Can you think of any reason the inside should look any better than the outside? Me neither...

Remove the master cylinder so you can look at it on the bench. Clamp it up in your handy bench vise after draining the fluid from the reservoir, and for safety reasons wrap a rag around the barrell to catch anything that sprays out. Then use a Phillips screwdriver to push in on the piston. My nickel says it isn't moving much if at all. The MC has a two-section piston inside, with a spring and couple little metal disk check valves, and of course some rubber cup seals wrapped around. The piston is aluminum, rides pretty snug in the steel bore. Not interference snug, but not normally much clearance around it. Any corrosion from moisture will take that space to zero or less, offering the symptom you describe.

What to do? New master cylinder. Get new grommets for the reservoir at the same time, and use some DC111 on those grommets to get them in a little easier and to keep water and future rust out of the bosses they sit in.

Carefully inspect the metal brake lines while you have them disconnected from the master cylinder. They may have a nice corrosion "lining" if they sat for a while exposed to air or contaminated fluid. If so, think hard about replacing those lines with new. You can buy lengths of CuNiFer brake tubing already flared, and bend them yourself. Our regular vendors may have other options up to new preformed replacements.

Take that same thought process to the rubber hoses at the calipers, and the calipers themselves. Corrosion and crud are the devils that ruin hydraulic components. Internal corrosion in components can't really be stopped once it's started, unfortunately.

GlenL

There must be a leak somewhere. Look for drips. It doesn't just evaporate through the rubber lines or any other "innocent" way.

It's not unusual to find that no fluid comes out when bleeding if there has been a lot of air introduced into the system. A good time for a power bleeder. I'll just press hard and fast on the pedal and it'll start something. Not advisable if the master is old.

The "shoes" are in the parking brake.

I suggest jacking the car and seeing if each wheel is braking when the pedal is pressed. That would be instructive on where the air is and any master cylinder failure.

Sounds good.

Maybe the booster isn't so good. The vacuum lines to it are under vacuum all the time.

You might have air there and it's not opening the clutch wide enough. A bleed there is a good idea in any case. Many posts on that. (many, many) Starting with an ordinary pedal bleed may be good enough. My go-to for this is to remove the slave and then force the plunger all the way in while doing the bleed. Tricky. Without good team coordination, the whole slave plunger will spit onto the floor.

James Bailey

Your brakes right front left rear are on one piston. Left front right Rear on the other....has two proportioning valves on the firewall to adjust rear brake bias.