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clutch pedal falls during low RPM operation

So, this should be easy but I am confused. When slowly accelerating or at low RPMs (2000-ish) the clutch pedal will fall about 1/2 way of fully depressed, takes about 2 seconds max. I use my toe to bring it back up & all is well. This is random as in it does not happen every time. I do not have hydraulic leakage issue. Clutch is fresh & has good release action, even at 1/2 pedal height.

It seems as if there is a vacuum that is drawing fluid from the master cylinder into the reservoir, this would imply a vacuum applied to the reservoir itself.

If this is the case then is vacuum from the intake/brake booster affecting air pressure in the reservoir?

Any reality check for my symptoms?
Old 07-12-2018, 12:25 PM
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9FF 9FF is offline
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That sort of problem is usually air in the system. You can disconnect and cap the vacuum line to the booster to eliminate it from the equation and see if the problem persists. Careful though as brakes will work the same but will require considerably more pedal effort.

Btw have you ever changed the line from the clutch master to the slave? It becomes porous over time and allows air in and fluid to sweat out.
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Last edited by 9FF; 07-12-2018 at 12:38 PM..
Old 07-12-2018, 12:36 PM
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Problem seems tied to low RPM only, no spongy feel in clutch application that would indicate air in line from master to slave. No leaking fluid noticed. Pedal actually is pulled down to 1/2 released position and this happens within 2 seconds once it begins to fall. This does not seem like air in the line which would be constant issue regardless of engine running or not.
Old 07-12-2018, 08:25 PM
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Yes I understand what you are saying, it’s just that there is no correlation between rpm and the clutch pedal and even if some weird vacuum issue was causing this, you can eliminate that like I previously mentioned.

The clutch is very simple: shared reservoir, master, slave, hydraulic pipe and cantilever spring. When there is air in the system it doesn’t generally feel spongy like a brake circuit, it causes the pedal to drop instead, a function of the air and the spring. Maybe your spring is adjusted so when you get some low rpm vibration the cantelever spring action causes the pedal to drop.

Also on the air/fluid leaks, even if you don’t see any leaks the system could still be leaking and introducing air. The intermediate hydraulic pipe between master and slave has a rubber section (photo below of one I’ll be replacing later today), the rubber degrades and “sweats” fluid on the surface of the rubber, you probably won’t see any actual leaks but trust me it is and air is getting in. Also the internal seals can wear on the master and cause internal leaks which can cause cavitation and introduce air into the system, again you might not see any visible signs of a leak.

I would disconnect the vacuum just to eliminate it as a problem, then look carefully at the cantilever spring action and adjust it if necessary. If the spring adjustment fixes it and it comes back after a while, that will confirm the leak I suspect you have.

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Current: 1986 951; 1986 944; 1986 944 (Track); 2000 996 (Cab); 2005 CL55 AMG; 2009 GTS V8 (6-Speed).
Past: 1972 RS1600; 1983 E24 M 635 CSi; 1985 308QV; 1985 UR Quattro; 1986 944; 1989 M635 CSi; 1990 560 SEC AMG; 1994 E34 M5 (Track); 2001 461 Puch G; 2002 E46 ACS3; 2006 X308 XJR; 2010 LP560-4 Corsa.
Old 07-13-2018, 04:58 AM
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Well, OK. I'll suspect the line. Weird that it is random in its action. I do understand the mechanics of hydraulic clutches. Kinda hard to imagine air passing through the wall of a hose under constant static & applied internal pressure.

Thank you for continuing to help.
Old 07-13-2018, 08:28 AM
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Do you have the pedal free play adjustment correct?
Old 07-13-2018, 01:38 PM
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Free play good.
Old 07-13-2018, 02:22 PM
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Quote:
Originally Posted by 1QuickS View Post
Well, OK. I'll suspect the line. Weird that it is random in its action. I do understand the mechanics of hydraulic clutches. Kinda hard to imagine air passing through the wall of a hose under constant static & applied internal pressure.

Thank you for continuing to help.
Even though it looks like a single contiguous resevoir, there is a small chamber which hugs the front part of the brake fluid resevoir that the clutch hydraulics pull from. This is what makes the process of bleeding the clutch hydraulics alittle difficult without a positive pressure bleeder- because the brake fluid resevoir needs to be completely full to overflow into the small chamber for the clutch hydraulics. Your hoses could have air in them and it would be really difficult to tell. I experienced a similar issue due to degredation of the blue low-pressure hose between the resevoir and the clutch master.

I am not offering this as advice, because its probably overkill, but if it were my car I would replace the master, slave, the blue hose and the high pressure fluid hose, unless I had already done so.
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Old 07-17-2018, 09:13 AM
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