V2Rocket

Before anyone tells me to bench-bleed etc...the master cylinder+slave cylinder + line between them have been on my car for ~10 years already.

In the last few months been experiencing on and off a weird issue with clutch pedal.

Most of the time the clutch pedal is firm and predictable as it should be.
Once in a while (more often lately) the pedal is very soft but still functions to change gear when given full pedal travel (basically all the way down).

-- If I am driving and feel the soft pedal and pull it back up, most of the time it is firm again.
---- If I wait more than a few seconds to touch the pedal again the pedal gets soft again. I mean soft like I can push the clutch pedal to the floor by hand with the same sort of force you'd use to open a lightweight house door.

Sounds to me like the classic (brake) problem where "if you pump it and the pedal gets hard there is air in the system". But aside from topping off the fluid the system has not been opened in 10 years...

So how would air get in? I am assuming it's in the master cylinder somehow but don't know for certain. I have not yet attempted bleeding by vacuum or power bleeder, but am just curious to possibilities of how I got here.

odonnell

A leaky seal in the slave or master could do it. When the piston pushes fluid out and then retracts, it could also suck in air around the seal as it does so, because of the vacuum created.

V2Rocket

I should add there are no external fluid leaks, anywhere, to speak of.
Not under the dash at the master, not at the master in the engine bay, not at the slave inside or outside the bell housing, clutch line is dry, no drips anywhere.

mrgreenjeans

the fluid is an attractant to moisture by it's nature. It absorbs water from the air, boils it when the fluid is hot at point of braking, moisture leaves and a bigger air bubble replaces the volume of moisture there in the first place. After time, it collects in the low points and the clutch bleed valve is the lowest point in the system which is a combined clutch and brake fluid chamber.

The area then becomes a repository for grit, rust, and contaminated fluid to continue to corrode the interior of the line and all components; the process becomes somewhat self perpetuating until one doesn't have any clutch actuation at all.

One of the reasons Porsche recommends in their Owner's Manual to flush out the fluid at minimum every two years and replace with a good quality, high performance brake fluid.

By your own statement you have only added fluid, not replaced it. It is about 5 times overdue for a complete flush and refill, but am going to guess the components inside are so gritty and corroded already, your master and slaves are goners.

Best wishes on your replacement of each. And the fluid too ..... ( gotta be all new. Recommending a high temp, low moisture retaining fluid like Wilwood or ATE Blue to do the job )

extanker

^^^^this

Tiger03447

DEAR MGJ: could dot 5 silicone fluid be used in this as well as the brakes? Higher boiling point and no water absorption..but harder to bleed for sure. As if we didn't have enough bleeding problems already with brake and clutch..:>( ...

Tom M'Guinn

If air is getting in, then fluid is getting out. Sounds like you have a leak. You didn't mention the carpet area behind the clutch pedal -- have you checked if the carpet is damp there and/or any other signs of leaking into the cabin there? That's a common leak point and is commonly overlooked until it's a real mess.

V2Rocket

Tiger - no, DOT5 is junk, and incompatible with our systems

Tom, no leaks anywhere, at all.

Tom M'Guinn

You checked under the carpet, under the clutch pedal? If air is getting in...

odonnell

Mine leaked out of the slave and dripped through a large notch in the bellhousing below the starter. With the engine running it could be easily whisked away or atomized before showing up. It also could be sealing fine until the piston gets to a certain point in the bore.

Another possibility here is that air can get in as described in my other post. No fluid is necessarily leaked, the air would change its position with use and lead to inconsistent feel.

V2Rocket

Tom yes pulled up carpet, felt around the MC where it comes through the firewall, felt the rubber boot, etc.

Michael, car stone cold, cycling the pedal a ton of times to get a feel for the...feel..., then checked MC and SC, stuck finger inside bellhousing as best I could, no fluid coming out, anywhere.

Everything bone dry.
I have read that it might be possible for an "internal seal leak" on the MC, that the pressurized fluid could be blowing past an internal o ring to a non-pressurized section of the bore (the feed side from the reservoir hose), had GF push pedal while I watched fluid level, no discernable change.

87944turbo

I had a seal on the Proportioning Valve leak and the brake fluid was making it's way onto the exhaust. Smelled absolutely awful. Mentioned it b/c leaking onto a hot surface can be a culprit for mysterious fluid disappearance, but if this were the case, you would absolutely know it, unless of course you're "nose blind". : )

Tiger03447

My bet, after reading all this, is that the fluid is bypassing somewhere inside the cylinder. As the rubber cups wear, they don't flare out as much as they did when they are new, and pushing fluid better...sounds like it's time for a new cylinder or at least a rebuild of the existing one. I would opt for a new one..because the bore is straight without any waves in the walls, which you might get when rebuilding the old one..due to honing of the cylinder, etc.$.02 worth..BTW..how much hassle is it to replace the clutch master, in these early cars? I've got a new one..so was just wondering..:>))

GPA951s

Had the Same thing happen to me YEARS ago... It kept Getting worse, To the point where I lost the clutch coming out of work.. Had to Wait till rush hour was over.. Start the car, Push it to get it rolling, and toss it in gear.. TIME all the lights.. and drove home 25 miles with no clutch.. Once I was rolling, no prob.. Anyhow.. The Slave turned out to be bad.. And I remember it happened on my Audi years ago as well.. Found the "Valving" washers on the outer diameter had "Sharpened" themselves form years of going back and forth on the bore.. they got so sharp on the O.D. that it tore the inner seal that it pushed against. -- No Leaks.. Only Internally.

MAGK944

^^^This. If you don’t have an external fluid leak then it’s likely leaking internally through the seals in the clutch master. If that internal seal is worn it may only intermittently transmit hydraulic pressure to the clutch slave, might have to pump the pedal a few times to get the clutch to operate.