I'm confused about brake pressure. I have an 82 928 auto that needs pads. Recently my brake pressure light came on and the pedal went to the floor. I've checked the archives and tried pumping the brakes with the ignition off and it slowly goes to the floor . I'm not leaking fluid from any tire, nor is any in engine bay (master cylinder). I will be buying pads first, but how do I diagnose master cylinder, pressure regulators or brake booster? Thanks in advance.

 

If the pedal goes to the floor, you have a leak. Check the floor board inside, just under the break pedal. The fluid may be on the inside of the firewall between it anf the rug.

 

If the pedal goes to the floor slowly the leak may be IN the master cylinder and not external. The piston seals inside the cylinder may be bad.

 

A little under a year ago, I had the same problem with my 928. It showed the same symptoms and at no point could I find a leak anywhere. I asked many people on this site and I received alot of very good information, tips and ideas of how I should take care of it.

In my attempt to remedy the situation, I replaced (new master cylinder, all new brake lines, pads, new rotors, abs sensors and dampeners) almost everything brake related. I figured that the car has 130K miles on it and it couldn't hurt to renew this part of the car.

I finally found out what caused the problem. The seals in the front calipers had developed leaks around the pistons. You could only see this when the calipers were removed. I ordered a rebuild kit from Prospeed Motorsports and had my mechanic rebuild them for me. I bought a Motive Power Bleeder and a set of Speed Bleeder valves along with a good supply of brake fluid (in the contrasting color to the stuff that was in there) and did the rest myself.

The brakes work great now and I learned alot about my car in the process. Hope this may help you out.

Rob

 

Gretch, I've checked under the brake pedal and it's dry. I'll check all calipers when I change pads and probably order a master cylinder. Thanks all.

 

A leaking m/cyl will leak into the booster - pull the vacuum hose off the booster, and feed semi-rigid wire downwards through the hole, until it hits bottom. remove and check for fluid. Anything more than maybe damp, you have a leaking cylinder. When the booster fills up enough the brakes will get VERY hard, not work at all well, and next the engine will suck the fluid into the intake, and produce CLOUDS of white smoke...

jp 83 Euro S AT 45k mi

 

JP... sounds like you've been there and done that.

 

Thanks Jpitman2,

I'll check the booster this weekend...

 

If the brake pedal will slowly sink under moderate pressure, the most common reason is leakage past one or both of the piston seals in the master cylinder. This will normally NOT create any visible leakage, nor will it cause leakage into the booster. All of the leakage is internal to the master cylinder.

Only cure is replacement or rebuild of the master cylinder.

 

VUnits,
You reading this thread?
Between those God-Awful lock-ups, spin outs and the strange smoke events/external leak -inner fender well under the M/C you may have a cure all here.