Porsche Repair: porsche clutch slave cylinder, clutch slave cylinder, brake master cylinder

Question
My son purchased a 1987 944 no turbo.  Recently the clutch petal went to the floor and I brake fluid under the car.  Removed the starter and saw fluid coming off the slave cyl.  Inspected all lines and hose saw no leaks.  Pulled slave cyl and it looked ruff at the shaft with signs of fluid comming out.  Installed new slave cyl from the porsche dealer and have been bleeding the daylights out of it.  I have been using a pressure bleeder but all I get is all with a little fluid. I read up on the issue before doing the job.  Think I have a bad master clutch cyl too?  I am wondering in which direction they work.  It would appear that the master would drive the slave and that's pretty obvious given the names but I'm also curious if the brake master cyl can have any impact on the clutch master other than sharing the same fluid tank.  Thanks for any input you may have.

Answer
Hi Phil,

This is a curious situation...no doubt created when the clever German engineers decided there would be money to be saved by having one, not two fluid reservoirs.

Here's the way to think about it.  I'm going to try to be precise with the naming convention:

The brake master cylinder and the clutch slave cylinder share the same fluid reservoir, meaning that the brake fluid in the brake master cylinder's fluid reservoir flows via gravity to fill the clutch slave cylinder.  That is their only connection, any brake pedal action has no effect on the clutch slave cylinder.

But what does affect them both is when one or the other is replaced.  That means unwanted air is introduced into both the clutch and the brake hydraulic systems and so both must be bled to complete the job.

That's the concept.  Now...one step you should have done but may have overlooked is to "bench bleed" the clutch slave cylinder.  This is the same step one would take if replacing the brake master cylinder, as it helps to remove any air in the cylinders themselves before they are mounted.

Now in practice, there is virtually no way to mount the cylinders with fluid in them, or without fluid dribbling out, right?  So...one method would be to simulate the bench bleeding after the slave cylinder is mounted:

1.  Temporarily abandon the pressure bleeder, OK?
2.  Make sure the master cylinder reservoir is topped off.
3.  Run the bleed tubing (hopefully a clear tube to more easily see fluid vs bubbles) directly into the CLEAN jar filled with a couple inches of new brake fluid.  
4.  Barely crack open the bleed valve and have you "assistant" pump the clutch pedal while you watch the bleed line at the jar.
5.  With fluid above and below, the clutch cylinder will push out fluid and air on the downstroke and pull back fluid on the release stroke.  Let's say one dozen full strokes should be plenty.  Eventually, all the trapped air will be removed from the cylinder itself.  If you still see bubbles after a dozen strokes, something else is going on.
6.  Close the bleed valve.

At this point you can return to your "normal" bleeding process, with this modification:

Since you will be bleeding the brakes too. and the guideline is to bleed the longest lines first...forget the clutch till you bleed the brakes first.  When the brakes are nicely bled, the clutch should bleed out so quickly your head will spin!

Have fun.

Dave