brake mystery - mc failure?

[grahamkissack]

I have just finished rebuilding all my calipers, replacing all brake hoses and installing new pads. After bleeding the brakes 7 (yes seven) times by vacuum and wife in the car, my pedal still drops about 4 inches before I get any resistance. My local expert says that 911sc master cylinders hardly ever fail but I think mine has. What's the total throw in the master cylinder before it reaches its max...does that relate to my 4" pedal travel?

Thanks, Graham

[911pcars]

This sounds like a common situation after a fluid change. While bleeding, the pedal is pushed down to the floor beyond normal brake travel. Inside the master cylinder, the piston pushes the seal over debris built up in the cylinder and tears it. This causes fluid to bypass the seal and piston resulting in loss of pressure.

Unless there's something wrong with your vacuum bleeding method (not the most effective method, btw), you might need a new MC. Lots of folks like the Motiv pressure bleeder; I prefer gravity bleeding. Either way, the pedal stays put but fluid (and air) exits the bleeder screws via air pressure (10-15 psi) or air pressure (gravity).

Hope this helps,
Sherwood
http://members.rennlist.org/911pcars

[911SCfanatic]

Lots of threads on this one. My recommendation is try some other stuff before making the drastic move of replacing the mc. I just did this a few months back. For me it was the pads were retracting due to new piston/caliper seal to the point that the pads were over 1mm out of engagement with the rotors--thus the long pedal travel. I did a bunch of hard braking maneuvers from about 35 mph, which moved the pistons relative to the seals to where they should be. Problem solved.

[chrisp]

Doesn't a bad MC (I have never had one) feel like a pedal that slowly travels all the way to the floor as fluid flows by the damaged seal?