Master Cylinder Replacement

Master Cylinder Replacement

> I'm getting ready to replace my brake master cylinder, and I was hoping
> to learn from anyone who has experience with this, anyone who can
> recommend a procedure, or anyone who could warn me of typical Porsche
> tricks and gotchas. (snip) Yet I've heard that this
> is one of the hardest jobs to do on a 928.


The brake master cylinder replacement is straightforward so you don't need to dread it very much; it's the clutch master cylinder that can be a major pain to replace. I've had the brake MC off my '83 a few times and I think the worst part of the job is making sure that the brake fluid resevoir reseats properly in into the MC. The reservoir to MC connection is a tight fit and there isn't much room beneath the fender (which has very sharp edges!) to get leverage when reseating it. Louie Ott's son was apparently able to replace the brake MC without removing the resevoir before removing the MC itself, so that would make the job easier but I haven't tried that yet. Since you're replacing the MC anyway, it really doesn't matter whether you empty the resevior by sucking out the old fluid or by pushing it through the calipers. Normally you wouldn't want to push the piston in the MC all the way to the end of its travel because you could tear the seal, but that's irrelevant when you're installing a new one. Drain the fluid from the clutch side of the resevoir by disconnecting the blue line and holding a catch can underneath. After replacement you can use your pressure bleeder to finish off the job.

You may want to consider the age of the flex brake lines between the brake calipers and the body when replacing the brake MC since you're going to have to rebleed the whole system anyway. The flex brake lines should have a production date painted on them, and if they're original they may be worth replacing.

Jim Stadter
'83 928 S 5-speed (U.S. spec)
'88 928 S4 5-speed

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