Dads928

Master cylinder pistons are likely worn allowing fluid to bypass them under sustained moderate pressure. You need a new master. Symptoms are similar for all master cylinders. If its the rubber brake lines, you will feel it under hard braking as well, the master will feel godd under sudden braking, it just won't hold under moderate sustained pressure, that's the master pistons.

WallyP

The problem is usually called "by-passing".

As the cups/seals on the master age, harden and wear, they get to where they don't flare out to seal perfectly unless there is some hydraulic pressure to help them flare or push back in the seal grooves. If you press harder when you fell the pedal sink, it usually causes the seals to make better contact, and the pedal will go back to normal. If you play with it a bit, you can usually put just a tiny bit of pressure on the pedal, and cause it to slowly sink completely to the floor.

Only answer is a new or rebuilt master cylinder.

Dennis Burford

Agreed. I had these exact same symptoms on a Jag XJ6 I had. Replacing the master solved it. One other note, on the Jag it only happened above 70 degrees farenheit, when cooler there was no issue.

danglerb

Mine didn't slowly sink, I would push on the brake, come to a stop, lower the pressure with my foot, and sometimes a second or so later with the same light pressure pedal would drop quickly like an inch and go firm again.

tailpipe

just make sure the booster isn't full of fluid when you change the master if it is just suck it out with a vacuum pump been there done that

danglerb

I've heard water in the booster can be a real pain to sort out, but its ok to just hook up an AC type vacuum pump?

kevinlieb

Update on this issue:
I bled all the brakes and it seemed much better. For a while. I bought a new master cylinder but never installed it since everything seemed to be fixed.

Now today, around 1 month since the bleed, I was driving to work and the pedal nearly went to the floor. I pumped the brakes and it came back up but at the next stop light the same thing. Scared the crap out of me. With the engine off if I push the pedal I can feel it creeping towards the floor.

I checked and there is enough fluid in the master cylinder, so no sign of a leak or just open bleeder. Also no fluids dripping out of any caliper that i can see.

When I press the brake pedal with the master cylinder cap off I can see fluid go up and down. Is that normal?

Did my master cylinder just completely fail on me?

Anyone know a good Porsche mechanic in the SF Bay Area in case I don't want to attempt this fix myself? Family and work pressures makes time for tinkering with the Porsche difficult to come by...

G Man

Nicole took her 928 to Rennshop in Santa Clara when she was having trouble with her brakes and they got the issue sorted for her quickly. Maybe she can give some input on them if you contact her. I would not drive the car until the brakes are fixed. It would be a shame to wreck it before you really get to drive it much.

http://www.therennshop.com/

kevinlieb

Well got the car towed home today. Hope to take care of the master cylinder this weekend. Seems like a simple enough job...

borland

Make sure you prime the master cylinder with brake fluid before installing it. That will make bleeding it much easier than with it already installed.

http://www.ehow.com/how_6177575_prim...-cylinder.html

Before starting, take note of the reservoir support bracket so you re-install it correctly. Best to use a half-moon wrench for the fire wall bolt under the fender.

borland

Also, after you remove the old master cylinder, check the brake booster to make sure it is free of any brake fluid. Often when the master cylinder fails, brake fluid leaks into the booster.

kevinlieb

Thanks for the tips! I didn't realize how important the bench bleeding is.

dcrasta

Glad you finally decided to do the MC. I cant believe you risked driving around since march. Willing to bet you a case of Fat Tire Ale the problems will go away once you swap the MC out.

dr bob

For those playing along at home, the bench-bleed procedure described above is mandatory fpr any MC where there are two separate pistons separated by a check-valve inside the bore. That description covers later 928 master cylinders nicely, and applies whenever air has been introduced into the MC. Even a small amount.

If the MC is installed when you let air in, you can play with ramps, curb, jacks, blocks, etc to get the installed MC level. The 'level' is important so there are no air pockets in the top front of each cylinder bore.

In the absence of the plastic fittings and hose described in the procedure, use a short section of regular metric metal brake line from your better local parts store. The steel lines come with flares and fittings on both ends. Cut the line in half, and bend the steel tube carefully so the cut ends sit in the top of the reservoir. If you want to see the bubbles, cut the lines short and use a section of clear vinyl tubing to route the pumped fluid back to the reservoir. Then follow the procedure to push fluid and entrained air slowly through the cylinder and back to the reservoir.

kevinlieb

I did the master cylinder and everything is back to normal. Thanks for the advice all!

It is a messy, somewhat difficult job. The ratcheting box wrench really helped on the inside fender bolt for the MC. My advice for anyone who tries this: get the plastic fittings for the outputs of the MC with a hose that goes back into the reservoir. These help in the bleeding but also give you a way to block off these ports when reinstalling in the car. Doing it with the steel brake lines looping back into the reservoir is fine but it makes it to bulky when installing in the car. Some other way to block off the ports is necessary.