Need Help.... Clutch hydraulics losing pressure

[300SDog]

The car is '79 240D 4-spd.... 112k miles... just bought the car.... its been in storage for long time..... mice were living in the hood insulation pad.

Clutch works fine for limitted period of time... no radical slipping or chattering... but some vibration coming from throw-out bearing....

Gradually and slowly while driving straight (without shifting gears) on highway, the clutch pedal will lose pressure - eventually going dead to the floor...... causing me to pump the pedal to restore pressure to disengage the clutch.

Clutch slave and master cylinders have been replaced..... no apparent leaks in hydraulic lines... entire system has been bled and flushed out.

Any ideas??

[240 Ed]

Could be one of two things.

Either the master or the slave is faulty.

OR:

You need to back bleed it. Take a syringe (buy it at NAPA) and fill with brake fluid and put a vacuum line hose to connect the syringe to the bleeder valve on the SLAVE cylinder.
Open the valve and push the syringe contents in. (make sure there is no air in the syringe!) Should work.

[300SDog]

Thanks for the reply..... neighboring "tech" forum clutch/search suggests it could also be mechanical - specifically, loose or bent clutch fork causing over extension of slave cylinder.... hence air getting into the system.

[240 Ed]

the normal wear pattern and the resulting travel of the mechanical bits have been taken into consideration by MB, and trust me, they know tons about manual transmissions.
So I would doubt that this is a cause.

Because you said you had to pump the pedal to get the pressure back is due to either air in the system (likely) or bad hydraulic components.

I chased this same problem with a SAAB. Drove me nuts. I bled the system by pumping the pedal until I got a firm pedal, and having an assistant then turn the bleed valve while the pedal sank to the floor. BTW, you don't actually want to go all the way to the floor. This is the way you are supposed to do it.

Didn't work.

Used the syringe method and presto, PERFECT!

I think that this tool is referred to as a "master cylinder bleeder kit" at NAPA.

[300SDog]

Thanks for getting back to me on this..... Have you seen the MB plastic feeler guage for measuring clutch wear according to slave cyl. extension? ..... not that i'd ever use one

For me, vibration of throw-out bearing is tie braker causing me to go inside bell housing..... also combination of factors that include lengthy outdoor storage (old lady owner never drove it again after killing a deer on the highway 9 yrs. ago). Itsa '79 w/112k miles.

Am suspicious of rusted, weakened pressure plate & throwout bearing on account of outdoor storage and the liklihood she rode the clutch at red lights.

My theory is that slave cyl. (recently replaced) over extends itself on account of weakened internal components. Have also heard of pressure plate fingers getting bent and actually braking off.... causing similar pedal to floor/ pump to revive symptoms.

Terrific forums here.... have spent hours using search features - worth every minute.

Car goes on the lift tomorrow....

[ctaylor738]

If you end up having to resurface the flywheel because of damage, you need to replace the flywheel bolts and use the correct shims between the crank flange and the flywheel to account for the amount that comes off the 'wheel.

Good luck on the lift.

[300SDog]

Epilogue, if anyone searches "clutch" and finds this thread....

After first replacing master and slave cylinders.... problem of losing hydraulic pressure is finally solved after going into the bell housing and finding worn out pressure plate and throw-out bearing.

Yes, it appears that slave cylinder has limitted effective reach and over-extends when internal clutch components wear thin.

[240 Ed]

thanks for giving us the results of your hard work.

I would have thought that there might have been some indicators of wear from the clutch, IOW-lots of bad noise?

Glad your car is back on the road!

[Matty]

I'm bringing a 1971 220 gas out of storage. The clutch master cylinder leaked badly just before shutting it down five years ago. Now while replacing the master cylinder I decided to replace slave cyl also. While removing and replacing the slave I noticed there is pressure from within pushing out. Is this normal for this model, the clutch worked fine just before storage. I've enjoyed you earlier communicaton. Many thanks... Matty

[300SDog]

Mine did the same thing... am guessing this to be self-adjust mechanism. Btw, use DOT 4 rated fluid when you flush hydraulics... other fluids are said to rot rubber gasket/seals in cylinders.

Also bringing my car out of storage, I scrubbed brake calipers and hydraulic fittings with brake fluid using toothbrush, checked all rubber boots on suspension knuckles and basically soaked any rusty looking parts with WD-40.

[Matty]

Thanks for your quick reply, especially about the DOT 4 brake fluid. As luck would have it I already had the system back together and it is working just fine. I did have an adjustment on the master cylinder and as you said the pressure from inside the clutch must be a self adjust thing with the slave. I bought the DOT 4 brake fluid and plan a flush and refill with the better fluid this week end.

I did get the old girl running but all rubber items will have to be replaced because of dry rot. It will be a slow process because time and money will set my pace of replacement, but I do enjoy it.

Thanks again for your help..... Matty