vytenisb

Needed brake pads. Figured after 19 years, some rubber parts might need replacement. Rebuilt all four (4) calipers and replaced brake lines.

Bled brakes using an assistant on the pedal with me on the bleeders. Couldn't get the pedal hard. Tried tapping the calipers. No success.

Brought it into the shop. Their bleed wasn't successful either.Diagnosis: master cylinder damage due to piston travel in parts of the bore not seen before.

Brakes were fine before I started. I've bled brakes many times before using the two (2) man method. Thought talk of damaging the master cylinder was an old wives tale. Apparently not!

Pressure bleeder will be the only way to go from now on.

JCP911S

FWIW I recently had the rear calipers on my 84 rebuilt, and subsequently had a sinking pedal.... decidede I needed a new MC, but had to get home and go to work, so I drove it for a week or so, and the problem (aparently) went away....???

I am no expert, but it is possible that the rebuild brakes allow the MC piston to travel farther in the MC bore, and that causes seal leakage... repeated use polishes the new section of the bore and that solves it.

Just guessing....

Peter Zimmermann

To do all four calipers at one time is a HUGE mistake. Calipers require a break-in period of about 150 miles, a little less if you use Porsche assembly paste. You don't have an air/bleeding problem, you've got caliper pistons that can't move freely because their o-rings are extremely tight. Once they are broken in they will be able to push the pads against the rotors on the first pump. After the fact, the best way to do calipers is one at a time. All that you can do now is leave long areas for stopping, go out and drive around with one hand on the e-brake, and triple pump the brake pedal during every stop.
Pete

JCP911S

Pete.... ahhh.... good info Makes sense

vytenisb

At start, all boots were still intact, bores & pistons rust free, and no apparent wear. I used ATE assembly lube and had lots of fun getting the new rear caliper seals on. An online post led me to a PVC coupling piece to press them on. Thanks.

I defer to the years of experience, but have a question.

If any or all the piston calipers were stuck in position, why would the brake pedal go down further than normal (approx. 75% of travel) and pump up?

Stays solid with pressure after the first pump.

1987 Carrera Guards Red
74K Miles

Peter Zimmermann

The pressure still builds normally behind the pistons, it's just not sufficient to move the piston far enough to allow the pad to contact the rotor, which will happen on the second or third pump. As the o-rings stretch slightly and seat against the piston things return to normal, and max braking capability returns to normal (one pump). I first discovered this problem in the early '70s, and have resealed hundreds of calipers since that time. I've also seen cars come in from other shops with your exact problem, just give it a little time, but anticipate stopping situations carefully.
Pete

vytenisb

Thanks Pete. Next time I will do them one at a time!

Capt. Carrera

Very interesting Pete. I've never experienced that problem. No, I don't doubt you at all. It make sense. Then again, I've rebuilt all four calipers, bled the brakes, and then hit the track with no problems.

...I guess it's a YMMV thing.

imcarthur

Thanks for the tip Pete. I am resealing mine in the next couple of weeks.

Ian

g-50cab

Did the same thing with 993 brakes - rebuilt them - speed bleeders (x 8) all new soft lines .

Probably the best piece(s) of advice I received was tapping the calipers while bleeding and bleeding with the old brake pads then pushing the caliper pistons back in for new pads at the end. Like all brakes make sure you bed properly - make sure you give yourself plenty of room when testing.

Chris Holcombe

I rebuilt my brake system a couple of weeks ago, new pads, rotors, brake hoses, and rebuilt calipers and had a very similar experience. The car would stop ok but the pedal felt mushy until you pumped it a second time. I thought maybe some air was still in the lines but after a week of driving it the problem went away and the pedal is hard as a rock now.

Peter Zimmermann

Ian: Do yourself a favor, when you buy your seal kits order quantity one - part # 000 043 117 00 (available through your dealer). That's factory assembly paste for the internal o-rings/pistons. There are many assembly "lotions" on the market, Porsche's stuff is the best. It not only aids assembly, it speeds up the break-in process.
Pete

arcsine

You can minimize the mush-phase by setting up the pistons in the calipers to just barely allow the caliper/pads to fit over the disk. Basically get as close of a fit as possible.

Peter Zimmermann

g-50: Multi-piston calipers usually break in much more quickly than single piston units. The outer piston circumference seems to have a lot to do with the break in period. For instance, a 1993 C4 uses a 40mm piston and a 36mm piston behind each (front) pad. In contrast, a 1983 SC uses a single 48mm piston behind each (front) pad. Somehow, the smaller pistons, probably due to o-ring size, seat in more quickly than the early cars. We were amazed how good the first 3.3L Turbo we did stopped after caliper work. Regarding the Captain's experience - great! Yes, we did reseals at the shop, on occasion, that felt broken in by the end of the test drive. But that was atypical. The average car, using Porsche assembly paste, required a break in period of about 100 miles before confidence was sufficient to do the next one. Before I "discovered" Porsche paste the typical break in period was about 150 miles.
Pete

JasonAndreas

The Porsche assembly paste is Klüberfluid 9 R 100 and manufactured by Kluber Lubrication. The seal kits for the 964 and 993 come with a ketchup packet of the stuff but I'm not sure if that is the case with the seal kits for earlier models.