Clutch Master Hose and Cylinder

18 Jun 2020 21:55 #21303 by Rich928
To many the manual shift 928 is the highly desired model. The clutch system for the manual shift is hydraulically operated. The source of the fluid is the brake master cylinder reservoir. A hose from the brake master cylinder transfers the fluid to the clutch master cylinder.

My manual shift Porsche 1979 928 diagnostic warning light flashed “brake fluid” when I turned on the ignition switch. I did see a puddle of something underneath the car. I popped the hood and observed that the hose that furnishes brake fluid from the reservoir to the clutch master cylinder was seeping fluid. It had drained enough brake fluid from the reservoir to trigger the warning light. I refilled the reservoir, double checked that the brakes worked properly and drove to my Porsche mechanic, Dan at Exotic Motorwerks for repair.

The brake fluid hose is labeled 29 in the diagram above.

It is a difficult and time consuming job because to remove the old clutch slave master hose the entire brake master cylinder and booster must be removed.

The brake pressure regulating valves must also be removed.

The clutch pedal linkage must also be disconnected. Once all the items are removed, only then can be clutch master cylinder be accessed.

In the photo above, the hole in the firewall is where the power brake booster is located.

Porsche Hydraulic Fluid Hose Part Number 999 181 021 50

There is not much more work at this point to replace the clutch master cylinder if needed.

Following reconnecting everything, the brakes and clutch master/slave need to be bled to remove any air in the system.

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25 Jun 2020 06:32 #21330 by joejabq
Great summary, Rich.
In the hopes of saving someone some frustration at the end of replacing the master clutch cylinder, the one thing that I could add is a word about the bleeding process.
I thought that it might be good to tack this video onto your post.

I think that until you see a video, you don't really understand why pumping on the pedal doesn't get the air out of the lines like it does for brakes.
I remember watching in amazement as big bubbles got forced out though the top hose using the method in the video.

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25 Jun 2020 06:55 #21332 by Trainer
You guys never heard of a pressure bleeder. It is not a very expensive tool

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25 Jun 2020 08:03 #21334 by joejabq
Hi Trainer,
You are right. I think that the WSM actually suggests using a setup to pressurize the system at the top. I tried that. Unfortunately, my old car (and I suspect that many of these old cars) could not maintain the pressure long enough to do the job. In my case, pressuring the system turned up very small cracks in the top of the reservoir that let the air escape. The reservoir doesn't leak fluid otherwise.
I guess the point is that 1. Pumping the pedal will generally not work for clutch hydraulics (see video); 2. A pressure bleeder is the WSM suggested approach, but you may stress old parts; and 3. a gentler and yet effective alternative for these 30+ year old cars is to pump fluid up from the bottom.
We have choices, which is a good thing!

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