Brake Problem

plhorner · May 19, 2005

Just finishing off the brake re-fresh tonight and I've run into problems I think!!!!
I blead the brakes through using a pressure bleader (which I incidently built myself for £3) and I reckon I've got all the air out of the system, the fluid is running clear and bubble free. First press of the pedal and it sinks a little way, I assumed bedding the pads to the disc and it then firms up on subesquent applications. Anyway, 5 minutes later, press the pedal and it again sinks half way down before firming up on subsequent strokes. Have I either still got air in the system or alternatively a kanckered master cylinder? Incidently, the master cylinder was fine before I stripped the calipers and I've not disturbed it in anyway as part of the work, save for replenishing the fluid [8|]
ANy bright ideas?
I seem to remember somebody in the dim and distant past mentioning a similar problem but can't find the thread.

bones · May 20, 2005

Run with the idea of air in the system before binning the master cylinder Phil. I've found it can take a few bleeds to get all the air out.

plhorner · May 20, 2005

Yeah, I was comming to that conclusion....... I was out in the garage in my dressing gown at 1am checking a few theories as I couldn't sleep!!![

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Guest · May 20, 2005

Another theory if more bleeding gets you nowhere. The new seals may just be pulling the pistons back. If you have 8 pistons all slightly back in their bores, you are having to push them out before you get any braking.

Is the pedal firm after you pump it up (so to speak)? If so, then you don't have any air in the fluid, instead you have mechanical movement causing the pedal to be long rather than compressable air in the system causing the pedal to be squishy. A similar thing happens if the wheel bearings are a fraction loose. The discs then vibrate against the pad causing "pad knock-off" and giving you a long pedal everytime you brake. Happens on the track a lot - happened to me at Silverstone in April.

One thing to try is to take the pads out and with old pads or a thin wooden block in place push the pistons out of their bores a little then push the pistons back in only just enough to get the pads in. Alternatively, with use the seals will bed-in and the problem will go away.

RB

Chris_911 · May 20, 2005

Watching this thread with interest.

I now have all the parts to complete my brake overhaul, including new rear calipers. Wondering whether I will encounter a similar problem.

Phil - hope you get it sorted[

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plhorner · May 20, 2005

Arrrh, yes, that ties in with what RoyM was saying in this thread that I eventually found. http://www.porscheclubgbforum.com/tm.asp?m=54463&mpage=1&key=master%2Ccylinder&#54769
Sounds like a little bit of stickson in the seals, I looked when the pedal is depressed and the pad is firm against the disc, when released you can see a gap appearing (0.5mm). If that's happening all round, that would explain the long first stroke before the pedal firms up.