Best Pressure for Bleeding Clutch Master Cylinder?

73ess · 01-06-2007, 07:56 PM

I've just installed a new clutch master cylinder in my '88 Carrera. I've got the rear of the car up on jackstands. Had my wife help me bleed the clutch line today using the manual method (open bleed screw at slave, push down on clutch pedal, tighten bleed screw, lift up pedal, etc...) with no luck. Pedal falls easily to the floor.

I'm going to use my Motive pressure bleeder next. I read one post stating that filling the brake fluid reservoir to full and using 25psi as the pressure value did the trick. Just want to make sure that this is the correct pressure and that it won't harm any seals. The Bentley manual I've got just describes the manual method, so no pressure value is stated for the pressure method.

Any better method that folks know of to get that firm clutch pedal again?

Thanks,
Steve

Jim Sims · 01-06-2007, 08:37 PM

I've used 10-15 psi in my Motive without issue; at some higher pressure the seals for the reservoir to MC lines in the top of MC may fail or blow out. I assume you installed new seals for these lines when you put in the new MC.

73ess · 09:07 PM

Thanks Jim. Also, should the clutch pedal be in the up or down position when using a pressure bleeder? Read one post that stated it should be down.

Not sure what you mean by new seals? There are just hard line threaded fittings at the MC and the Slave. No seals, other than the seals inside the cylinders. I installed a new MC so would assume it has new seals.

Jim Sims · 01-06-2007, 09:19 PM

Failed to note you are doing a clutch MC; the seals I was referring to are on the older 911 brake MC's between the reservoir and the MC. Someone else will have to answer the question about the clutch pedal position.

Thomas Owen · 01-07-2007, 03:33 AM

From my experience the pedal was down while using the pressure bleeder.
Good luck -

73ess · 01-07-2007, 07:56 AM

Thanks Thomas. So place the pedal in the downward position and allow it to raise up during the bleeding process? Or should I hold it down with a block of wood?

SLO-BOB · 01-07-2007, 08:24 AM

When I bled my brakes and clutch I set the pressure at 20 psi. I also followed the advice of some and dumped the brake fluid right in the bleeder. This after I quickly emptied my resevoir doing it the "old fashioned" way and blowing myriad air bubbles in my brake lines. It really does work better that way and clean up is easy.

I'm no pro, but I believe the pedal has to be up. Considering you actuate a plunger that pushes the fluid, it seems to me that you want the peddle up, allowing for that chamber to fill with fluid. That's how I did it anyway and all is well. Please correct me if I'm wrong for future work.

Green 912 · 01-07-2007, 08:28 AM

15 psi is max. I vote that you keep the pedal Up and this is why. If the pedal is pushed in at all it will close off the equalizer hole in the clutch master and prevent fluid from flowing. Also, you should stop the fluid flow two or three times and pump the clutch vigorously then bleed some more. The clutch master and slave have high spots that can trap air and do not flush well. Pumping the pedal foams the trapped air and when you restart the flush it comes out with the fluid.

73ess · 01-07-2007, 08:35 AM

Ok. Thanks guys. I'll start with the pedal up then and pump the clutch, after each bleed. So how many times will I need to bleed the line, before I can expect to get a good pedal?

SLO-BOB · 01-07-2007, 08:42 AM

Quote:

Originally posted by 73ess
Ok. Thanks guys. I'll start with the pedal up then and pump the clutch, after each bleed. So how many times will I need to bleed the line, before I can expect to get a good pedal?

Just did mine once with the clutch up.

73ess · 01-07-2007, 08:45 AM

Just curious why the manual bleed method didn't work though? My friend says it should have just taken a couple of pump on the pedal. I guess my slave could have a leak at the pushrod, but so far, I'm seeing no evidence of leakage at the slave. Eventhough the pushrod is hidden, wouldn't I see fluid coming from the transmission somewhere, if there was a leak at the slave pushrod? Or maybe not?

Green 912 · 01-07-2007, 02:04 PM

Porsche clutch MCs and slaves are known for trapping air.

73ess · 01-07-2007, 08:28 PM

Finished the bleed. Used my Motive pressure bleeder set to 17psi. The main thing to remember is to open the bleed screw almost a full turn. I had it open about a half turn and thought that was enough. Nope, uncrew it about 3/4 to a full turn to really see the fluid move. Got a nice firm pedal again. My '88 Cab is back in action.