Squishy pedal after caliper rebuild

[Flieger]

Eric, thanks for the tip.

I actually prefer the location and travel of the pedal as it is now. It is positioned perfectly for my style of rolling my foot onto the throttle to raise the RPMs during downshifts and the extra travel makes them easier to modulate so I can carry more speed and load the right tires when trail braking into a turn.

[burgermeister]

Just for (useless) reference, at the factory a machine draws a vacuum on the brake system, holds it for a wee (to check for leaks), and then fills it with fluid under pressure. So the factory has no need to bleed the brakes - the whole operation happens at the master cylinder reservoir. Of course, Porsche may be different .... those crafty wacky Germans have so many unique solutions

[burgermeister]

I rebuilt all 4 of mine, and a new master cylinder just to be sure (I do like to stop). Had exactly the same issue - soft pedal, could easily push it to the floor, though car was driveable and I could lock up the fronts.

I did not bench bleed the master cylinder.

I vaccuum bled (sucked, pun intended), pressure bled (worked quite well), pedal pump bled (worked OK, wife was impatient), even applied a vacuum to master cylinder reservoir to see if MC had any bubbles (do NOT do this - it will suck air into the clutch slave cylinder, as well as one or 2 calipers ... dumb desperation move on my part, and as penance I got to bleed the clutch as well ).

In the end, it was the sticky pistons. 2 pads had about 1/64" + of gap to the rotor. Took the pads out, pumped the brakes twice, pushed the pistons back a bit until I could just slip the pads in, and the brake pedal has never felt better.

I suspect slight pitting of the pistons (not enough to leak brake fluid) may be at fault. The issue was with the calipers that had said slight pitting. If the problem recurs as the pads wear, I guess new calipers are in order, as I have not found the pistons separately.

Other advice: Put brake pads in ALL 4 calipers (even if the kids or friends interrupt you twice) before pumping the pedal repeatedly and wondering why it is soft. Otherwise, you get to redo some calipers twice. And, if your brakes pull or one side always locks for no apparent reason, check to make sure some yahoo (PO in my case, but I would not be above such things either) didn't put the piston notches in the wrong spot. Mine brakes dead straight now that that got put in order.

[greg bradbury]

I finally have a good pedal after months of bleeding. My pads were up against the rotors and I had put some old pads in for a while as suggested by my local workshop. When I put new pads in the rear I had to push the calipers back for the new pads. Now the pedal is good, why?

I have a theory.

During normal bleeding the air rises to the top as fluid is not being pushed through fast enough. Look at the pipes exiting the master cylinder and they curve up before disappearing down under the car. It's a long way to push the air down and eventually to your bleed nipples. Any pauses in the bleeding and the air quickly rises to the top again.

Solution.

Remove rear pads on one caliper and replace then with a piece of wood approx 1/4" thick. Then slowly ( very slowly) push the brake pedal down to push the calipers in to the wood. When this is done take a breather 15 minutes and let the air get back to the top of the lines.
Later on remove trhe wood and push the calipers cups back home into the caliper as fast as you can. This will push the air and fluid back into the MC resevoir. Then bleed the brakes again for the ....th time.

Give it a try. It worked for me.

Brad.

Give it a try if all else fails

[tomphot]

I'm having the same issue after rebuilding the front calipers.
When bleeding over and over again, since the air is most likely in the front, do I need to bleed the rears every time I bleed the front?

[Charlie V]

Same issue on my SC and a friends after rebuild. Bled the system 8 times each - lots of expensive brake fluid into the recycled waste oil. There was no air that came out of the calipers on the last 4 bleeds.

We speculate that the square caliper seals need to seat in over time. Because, after a couple thousand kms, the brakes firmed up and everything worked fine again.

[SpeedracerIndy]

Since I started this thread over a year ago, my brakes have been perfect. They just needed time to seat. They are now rock hard and good as new. I think I bled a total of 4 maybe 5 times when I replaced the calipers. 3 were right away, and then a 4th one after I had driven a few hundred miles.

[tomphot]

I just played with it a bit more - one of the pads was really loose on the rotor. I took it out, extended the piston and then pushed it back in just enough to make a very tight fit for that pad.
Did another 2 bleeds and it feels a lot better - not as firm as I'd like but hopefully once things seat better, 1 more bleed will take care of it.