924Board.org

xmcginness · Joined: 01 May 2004 Posts: 94 Location: Denver, CO

I haven't a clue what happened. Couple days ago went out, after not having driven her for about a week, pressed my foot on the clutch pedal and it dropped to the floor and didn't spring back. Reached down pulled it up, pressed my foot against it and again.... no resistance...it dropped to the floor!

Anyone got any diagnostic ideas?

Christopher

Chrenan · Joined: 15 Jan 2003 Posts: 3903 Location: Canada

Snapped clutch cable would be my guess.
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1987 951 - M193 Version for Japan

924RACR · Posted: Fri Dec 31, 2004 5:22 am Post subject:

More likely blown hydraulics, I'd expect a cable to die in use, not sitting. But he doesn't mention which kind he has (though this is the classic symptom of bad hydraulics).

Feel for a wet spot on the firewall carpet, wet with brake fluid...
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

xmcginness · Joined: 01 May 2004 Posts: 94 Location: Denver, CO

Sorry! She's a 1980 931. Does that help. I'll check the firewall carpet and report back later today!

Chrenan · Joined: 15 Jan 2003 Posts: 3903 Location: Canada

That would be a hydrolic clutch, not cable...
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1987 951 - M193 Version for Japan

Smoothie · Posted: Fri Dec 31, 2004 8:17 am Post subject:

As the seals wear, they'll get to a point where the clutch still works in warm weather, but when the temps drop, the seals shrink, so you end up with worn, shrunken seals that aren't in close enough contact with the cylinder walls and the fluid slips by. If that's what's happened, and while you're waiting for replacements or rebuild kits, if you need to use the car, you might get some more use out of it by warming the engine and also pumping the clutch pedal - (way quicker to do it by hand - first check the fluid level, then just throw the pedal down, pick it up, repeat - could be as many as 50-100 times, but it goes quick doing it by hand. Do it until the pedal starts to come back, then you can do it by foot a few more times) Once the pedal's back and the engine's warm, you should be good - take it on a short low speed drive to check it out so you don't get any suprises, and expect it to drop to the floor again each night until you have it back together with new seals.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Paul · Posted: Fri Dec 31, 2004 9:35 am Post subject:

Not to be a PITA, but:

Rubber expands when cooled, shrinks when heated.

In the case of our master cylinders, I believe they are more prone to leak because the rubber loses its flexibility when it's cold outside, not because it shrinks.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Smoothie · Posted: Sat Jan 01, 2005 2:41 am Post subject:

It's most likely true that the loss of flex has more to do with the failure than shrinkage, but I find the OD of the seals does actually shrink slightly when cooled and expand when heated. If rubber itself does actually expand when cooled, then the opposite effect on the OD of a hydraulic seal might be explained somehow by the cross-section shape of the seal - (ie. expansion of the material causing a distortion of the original profile resulting in the outer lip of the seal moving inward) - I don't know, just a theory and rubber's weird stuff, so you could waste a whole lotta time trying to predict what it will do. Nothing like actually measuring though, and I found the OD expanding by approx. .005" when moved from a freezer to roughly 75*F (always think twice before questioning someone with too much free time on their hands

). In any case, I'd have to agree that the loss of flexibility is the main culprit, but more important of course is the end result - that worn seals can fail when cold, then perform better on re-warming.

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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Paul · Posted: Sat Jan 01, 2005 3:12 am Post subject:

I agree that age and cold temperatures makes rubber less flexible.

Wonder what our seals are made of, neoprene perhaps?
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

CMXXXI · Joined: 05 Nov 2002 Posts: 1939 Location: Vicksburg, MS

Pull the inspection plug on the bell housing and see if hydraulic fluid pours out. If it does, it is without a doubt the slave cylinder. Is your fluid level adequate, or perhaps have you just accidently sucked some air into the clutch plumbing? Replacing either of the cylinders isn't hard, it just isn't very much fun (nor is bleeding it for the first time after a component replacement).
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'79 Eurospec 931

Smoothie · Posted: Sun Jan 02, 2005 10:10 pm Post subject:

I don't know if anyone else's tried this, but I found a way to reinstall the slave cyl and avoid the traditional pain in the A bleed process... Basically: bench bleed the cylinder, then attach the hydraulic line, then attach and bolt the cylinder to the bell housing (the piston gets depressed while doing this, forcing fluid up the line, in effect bleeding the line backwards) - follow that by hand-pumping the pedal 'til it comes back, then foot pumping 'til it's solid, and your done. It's shown here -
http://www.924board.org/viewtopic.php?t=11373
and mentioned here -
http://www.924board.org/viewtopic.php?t=11206
..and some stuff on the clutch master cyl -
http://www.924board.org/viewtopic.php?t=10391
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Paul · Posted: Mon Jan 03, 2005 1:22 am Post subject:

For less than $50, this makes it very easy to bleed our brake and clutch systems. I installed a clutch master cylinder dry then used one of these to bleed it. Didn't have to pump the pedal even once, worked the first time I pushed the pedal.

_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

924RACR · Posted: Mon Jan 03, 2005 2:11 am Post subject:

Agreed, but I've got an even better way to install dry.

I replaced the hydraulics on my 81, and so both were dry. I used my Mightyvac on the slave before bolting it in. Since it sucks fluid, rather than pushing the fluid through, it can be used with the slave plumbed in but unbolted. This allows you to tilt the slave by hand and get the best angle to let all the air out through the bleeder screw.

Perfect results first time, have not had to re-bleed the hydraulics. This compars very favorably with 944 owners who've had to bleed them many times over months to finally get a good pedal.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype