Brake Pressure Loss Issues???
 

My brakes on my 911T are not retaining pressure. I put in a new master cylinder and all of the wheel cylinders approx 4 years ago and everything worked great. Then about a year after that I dis-assembled the car for a restoration. As part of the restoration I pulled off the brake fluid reservoir and plugged the tubes that run from the Master Cylinder to the brake fluid reservoir, but left all of the wheel cylinders and brake lines untouched.
When I re-installed the brake fluid reservoir I filled it up and used a power bleeder to bleed all the brake lines, starting with furthest wheel and working my way to the closest. When I complete that, the brake pedal has resistance and there is pressure in the lines. Then after a short time all the pressure in the lines is lost. I tried bleeding the brake lines a second time and the same thing happened.
I do not see any leaks where brake fluid is leaking from.

Any suggestions??? Would the master cylinder go bad after sitting a few years during my restoration?

How old are the rubber hoses?

Is it leaking from the lines in the center tunnel?

Pull up your carpet and floorboard and check around the pedal assembly. Sometimes if the MC goes bad it leaks around the piston and drains inside the car around the brake pedalshaft. Before bronze pedal bushings this type of failure could destroy the plastic pedal bushings and require a pedal assembly rebuild. Good luck in finding your problem.

It was not clear in your post if you were losing fluid from the reservoir. If you are, then the above replies are certainly your first line of investigation. If the fluid level is not dropping, however, I'd suspect a bad seal within the MC.

To clarify, the fluid level is NOT dropping. And, there is no fluid dripping either by the pedals or anywhere else that I can see. Thus, perhaps the MC has a bad seal?? Would the seal go bad from just sitting?

Anything is possible.

But first, when you bled, did you pedal bleed method or just the pressure bleed?

Changing the reservoir appears to be the cause of the issue, are you certain you did no introduce air into the system? Not for nothing but maybe try reverse bleed from the brake light switch???

Jim

When I bled the system the second time I did both the pedal and pressure. I no doubt introduced air into the system when I removed the reservoir. When I removed the reservoir I simply loosened the small hose clamps below the reservoir and disconnected the reservoir there, then plugged the lines there to keep debris from getting into the lines, and that's all that I did. The brake fluid remained in the lines from the reservoir tubes on down.
I sure don't like the idea of buying another master cylinder again as I really only got ~ 6-10 months use out of the one installed, but I guess it's sounding like that is the issue?? Any other ideas?

When you did the pedal method, did you go all the way down to the floor when you bled?

If so that may have caused the damage of the seal?

Try to bleed again, I know it is annoying but I would try again before pulling the trigger on another MC Try the reverse bleed, you may be surprised, it works well.

What exactly is a reverse bleed and how is that done?

The idea is to bleed up to the Reservoir, Air wants to go up naturally.

http://forums.pelicanparts.com/porsche-911-technical-forum/642694-reverse-brake-bleeding-field-test-results-beware.html

I also used a different pedal method, I filled a long piece of clear hose with fluid and the other end to a jar. I placed the jar higher than the car's fenders. I opened the valve and started to press slowly and continued to do so leaving the valve open. I actually saw air bubbles go up the clear hose and into the jar.

I used a piece of wood to keep from going all the way to the floor.

You have to make sure the valve does not leak when it is cracked open a 1/4 turn.

Bad master cylinder.