I have a 1980 911SC that I just (yesterday) rebuilt all four brake calipers. I used german rebuild kits for the project. I also added stainless braided brakes as well. After I reassembled everything and bled the system with my handy-dandy Mytivac bleeding system, I still got the soft (very) brake pedal. I resorted to employing an assistant to manually bleed the system. She would pump the brake pedal three times and I would then release the pressure at the caliper. I started at the RR, LR, RF then LF. I repeated the process four times on each caliper. The brakes never firmed up. I then (carefully) drove the car to a Goodyear shop where I knew the boys in the garage (no real Porsche experience). They Bled the system and still could not get the brakes to firm up. Our thoughs then became the brake master cylinder is the problem! There are no leakes anywhere on the car. All calipers are dry. No leakes present in the boot. My thought now is to replace the master cylinder with a new unit. The car has 120000 miles and at the time of my purchasing the car it was in a miserable state. Any ideas or comments would be greatly appreciated!

 

Groovy:

Assuming that there really is no air in the system and the MC is OK, you are experiencing something that can happen after caliper rebuild.

The square-profiled piston seals are not yet positioned and sometimes this will result in a soft pedal for a bit. Just keep pushing the brake pedal and it will get better.

Now, if the pedal is soft and slowly sinks to the floor under heavy pressure, your MC is likely bad and needs replacement. Its not unheard of for these to fail at that mileage.

 

Wait!

None of us can really know exactly what you mean by "soft pedal". My 78 brake pedal seems to have two "stages". The first inch or so of travel is quite soft, then after that it stiffens up considerably. Braking action has always been stellar.

My mechanic says this is SOP for the SC series.

 

Just to add to Steve's post, I've seen too where on a complete rebuild, the rebuilt calipers are put on with the pistons pushed all the way in. It can take a hundred miles or so to get them wiggled all the way out, and to seat with the new seals. Or you can just sit there and keep pressing on the brake pedal.

On your master cylinder, you may have set yourself up to replace it in the near future. Using an assistant to push it to the floor in bleeding pushes the rubber seals over parts of the master cylinder interior that don't normally see movement, where corrosion builds up over time, which can rip the seals. Given the high mileage you have and assuming it hasn't been replaced before, you may need to replace it sooner rather than later. Steve mentioned how to test the master cylinder condition.

 

In response to Steve's post, my brake pedal goes about 3/4's the way down to the floor when I first push down on the pedal. When I press down again, it goes down about half way and on the third time in a row it goes to a bit less than half way. It feels wrong....

 

By the way, thanks for the comments! I do not take this forum lightly or for granted. I feel it is a very important resource for all Porsche owners to share their ideas and knowledge! Proud to be a member! Thanks again!

 

Groovy:

If you push on the brake pedal (after you pump up 3 times), does it slowly sink to the floor under hard pressure? If so, the MC is bad.

Just keep pumping the pedal a lot, if you pushed those pistons back when you rebuilt them. It really takes a while.

 

Are you sure you didn't get the calipers mixed up left to right? bleeding screws should all be at the top. don't ask me how I know

rjp

 

Randy,

Good lookin' tail on your SC!

 

Groovy,

I have had this happen with reconditioned calipers but not new Porsche sourced calipers - I have also reconditioned using the kits supplied with new rubber components but not had this problem. Steve has it right - the seals are not sitting in the correct place, the pistons are too far back in the calipers. My solution was to pump up the brake pedal to get good hard feel and than brace the pedal in the "on" position against the seat runner/frame with a peice of wood. Left on for a few days my pedal came back to about half, after a second dose all was right with the world (the second time I left it about 2 weeks). There is another alternative - strip them down and when iserting the piston only push them far enough to take the rotor and pads.

PJC

 

Thanks Bill.. Found it in TX and it needed a bunch of fiberglass work....I'm looking for a factory early Carrera tail (a real one) if anyone has one for sale..

rjp

 

Randy,
Are you talking about a Duck Tail from the Carerra RS?
Let me know. What would you be doing with your whale tail?

Thx
-Jason

 

Hey Guys!

When I rebuilt the calipers, I DID push the pistons all the way back! I remembered when I did this project previously, I had a bit of trouble inserting the pads after I rebolted the calipers. This time I thought I would make it easy on myself (hahahaha....hmmm). The situation is now I can drive the car. It does stop when I apply the brakes. It still does not feel correct. There is much play in the brake. (I have only driven the car on quiet roads near my house). I hope it is only the pistons being back too far after the rebuild. I will try pjc and Steve's recommendations will let you know what resolution I come to! Thanks again!

 

The tail I'm specifically after looks exactly like what's on the car now, but it's a Factory piece. Has a metal frame inside.

What's on the car now is a POS aftermarket piece that, honestly belongs in the garbage. It was money wasted trying to salvage it. I'm anticipating replacing it sooner or later.

Lesson learned. Avoid aftermarket fiberglass.

rjp

 

I just did mine. I heard a trick on the Pelican board about bleeding with the old pads in place, then put the new pads in - Knock on wood - but it worked and I have a firm pedal.

I just rebuilt the fronts but changed out the flexible lines too.