Terry Lefco

I just replaced my brake fluid in my 1996 C4 with Super Blue in anticipation of a track event, but now I can't get a firm pedal. I bled the brakes three times, both with a pressure bleeder and with a helper depressing the pedal as apprpriate. We,as well as aPorsche service tech who was there, agree that the pedal is hard before ignition, but then it seems softer and sort of kicks back in response to pressure. The car seems fine in street driving but I don't want to run out of brake at the end of the front straight at the Glen.
Ideas?

Ray Calvo

"with a helper depressing the pedal as appropriate" !!!!!!!!!!!!

I hope it isn't what I think it is - a bad master cylinder!

I destroyed two of these using the old fashioned "Have Abbott pump the pedal and hold it while Costello opens the bleed valve" on my '76 911 in 50K miles. This method definitely forces the pistons in the master cylinder past their normal wear points, and prematurely destroys the seals. Results in a soft pedal, especially noticeable on initial pedal push during a drive; one or two additional pumps brings pedal to normal firm point, then feels soft again on intial push a few minutes later. Switching to pressure bleed resulted in no problems in next 70K miles on '76, in 120K miles on '84 Carrera, and 60K miles on 993.

Other thought might be some problem in the hydraulic brake boost system, if you say OK with engine off but system kicks back during braking while driving. Then again, this might be something in the ABS kicking in when front or reat ain't braking the way it should - which could point to the master cylinder again!

DEFINIITELY get it fixed before going to the Glen!!!!!

Robin 993DX

Another possibility is that you have air bubbles trapped in the ABS system. In order to bleed out the air bubbles out of an ABS system you need to go out and drive the car to get the ABS to fire up and then "flush" the brake system again. You may need to do this several times to get rid of all the air that's trapped in there.

Terry Lefco

Well, the consensus is a failing master cylinder so I'm just going to bite the bullet and replace it before the track. It's perfectly OK around town, even getting into ABS on dry pavement, but the pedal goes down alarmingly on sustained pressure, which is not good when you need to slow down fast.
Thanks for the helpful comments.

Ray Calvo

Terry, you didn't say this before ("pedal goes down alarmingly on sustained pressure"). By this, do you mean if you hold steady firm pressure on the pedal it continues to travel farther and farther down? From my experience this more certaintly points to a bad master cylinder -unfortunately.

Good luck!

Terry Lefco

Ray Calvo is right as usual - it's the master cylinder. Thanks, guys!