Ok guys, I need your help. I'm having some problems with my brakes. I have adjusted the rear venting clearence, rebuilt the master cylinder, installed a T in place of the proportioning valve and replaced all the fluid with new DOT 4. I have also checked all the hoses, and the look good, no cracks or anything, and there are no visible leaks anywhere in the system.

The problem is, I can't get a solid pedal. I have bled 3 times since rebuilding the master cylinder and installing the T. I bled once using a vacuum pump, once with a friend stomping on the pedal, and once with a pressure bleeder. I can't see any air coming out, so I'm assuming thats not the problem.

I'm stumped, and very frustrated. The only other thing I can think of is that the calipers need to be rebuilt. They don't look like they're leaking, but one could be stuck. Could a stuck caliper cause a soft pedal? And it's very soft, I can get almost all the way to the floor if I really stomp on it.

Any ideas? I'm getting really close to taking it to a mechanic, but I'm to cheap and to proud to do that. I'd much rather fix it myself.

Any and all ideas greatly appreciated.

er...try a new m/c? Any fluid squirting out anywhere?

One sticking piston will cause a spongy pedal.

I pull one brake pad out on one caliper and slowly have a buddy push on the pedal while I hold a screwdriver against the piston with a small amount of pressure.. I watch the piston..if it moves out easily..I push the piston back (with the screwdriver) and re-install the pad.. repeat with EVERY pad (not on the rear).

Just make damn sure you dont leave a pad out...

You should see each piston move out.. they should also move back in nicely without any issues. BE CAREFUL. You can pop a piston out (it wont hit you.. but it will soak you in fluid)

I have to use the procedure on the race cars when they have sat for awhile.. We call it "excercising the pistons".

Oh.. NEVER rebuild MC's... its your life.. buy a new one.


B

Hi,
I had a similar problem with my 911. It turned out to be the MC. Listen to Brad. Buy a new MC. I got the 19mm. Works great.
Cheers, Elliot

I have a feeling it may be a stuck piston. Assuming it is, what do I do to unstick it?

My explanation above will unstick them.

The problem is... if its a REAR piston stuck.. your going to be there forever. You cant push the rear pistons back in by hand.. the auto adjusters on the pads wont let you. You have to adjust them and adjust them.

Pray that its a front.


B

I agree with getting a new M/C in lieu of rebuilding. The 19mm M/C's are still affordable and will last a long time. I NEVER rebuild the master. While your at it, if you don't know the age or the history of your hoses, I would replace them too while you've got an empty system. Old hoses may look OK from the outside, but on the inside, they swell up shut after long periods of non use. Life of hoses are about 10 years with care under normal use. Less if the car sits. Brakes need exercising.
After a short drive with light brake use, put your hands on all the hubs. If they are hot to the touch,(you can burn yourself) then you have a problem with the calipers. If you suspect a particular caliper, jack it up and rotate the wheel. It'll be VERY difficult if it is seized.
Don

Try setting the venting clearance at .004.

D*mn! No wonder I was having problems... I was exercising the calipers

in addition to all those other suggestions - which were excellent - (and pardon me if this seems too obvious...) but just because there is no air coming out doesn't mean there might not be air still in there...

i did all that bleeding stuff too, once upon a time, and still had no pedal... but it turned out that the bleedscrew was not in the -exact- top of the caliper, and there was a big enough bubble above the bleedscrew to cause a problem.

try unbolting the caliper, insert an approximately disk-thick chunk of something between the pads, and rotate the caliper until the bleedscrew is absolutely the highest part of the caliper - then use wither the pressure-bleeder or friend method to force out any remaining entrained air.

"it couldn't hurt..."

excessive venting clearance causes this too, as previously mentioned.

Also try pumping the pedal several times until it is hard, than open the bleeders to bleed off the air. You may also have to bleed all the calipers several times to get the air out. Just take it slow and keep working it and the pedal will finally get firm.

I use a long stick on the brake pedal with the doors open when I bleed the rear ones. Put the end of the stick on the pedal and pump it by hand. Than I only need my wife or little girl to do the front ones.

Paul

Something that hasnt been said:

DO NOT pump the pedal quickly.. slow even strokes. If your "pumper" hits the pedal fast 5-6 times it scatters the air bubbles.

Also.. we have seen a slight benefit to tapping the caliper whilst the "pumper" is pumping the pedal. This will cause the air bubbles to rise to the bleed nipple.


We have been doing the one pump..hold...bleed....pump...hold...bleed.


B

and.......

your M/C can be bad, rebuilt or not.....I've known of brand new M/Cs failing right out of the box with syptoms excatly like yours