What has happened to my clutch?

[ukkid35]

AP Twin plate clutch with new Raceproved slave, relined friction plates, and skimmed flywheel

Works great for three hours, light smooth pedal action, no slipping when changing gear, or on inclines

Parked overnight outside, then within 20 minutes while still cold the clutch starts slipping, bite point moves higher, pedal pressure needed now greater

Slipping gets worse on the way home, so inclines start to become a challenge as the drive train heats up

Car parked up, left for a week, now the pedal is smooth, bit point returned to normal, not possible to provoke slipping

Presumably that rules out contamination, so what is going on?

[Boosted LS1]

Stiction in the hydraulics perhaps?

[gazza285]

Boosted LS1 said:

Stiction in the hydraulics perhaps?

Master cylinder not quite fully retracting perhaps?

[Megaflow]

gazza285 said:

Boosted LS1 said:

Stiction in the hydraulics perhaps?

Master cylinder not quite fully retracting perhaps?

That would be my bet. I saw a Mondeo with a clutch pedal stuck on the floor, and when it was stuck the clutch was open as well. A garage replaced both cylinders and the fault was still there, turned out to be the flexi line to the space breaking up internally and creating a sort of one way valve inside itself.

[ukkid35]

Boosted LS1 said:

Stiction in the hydraulics perhaps?

That's what happened the previous month

While the gearbox was out I decided to replace the slave seals as preventative maintenance, the slave was working fine at the time

I used a seal kit that is sold for the OEM slave, and they seemed to be the right size, although it was somewhat tighter when assembled carefully with red grease

I assumed it would 'wear in' and checked the action of the clutch with the bell housing fitted, but before attaching the gearbox, it seemed OK

However when I got to test drive the car, the clutch action was very slow, with the clutch slipping for a moment before engaging

As the drive-train warmed up, the action became quicker, and I became less concerned

It was good enough for me to be able to do a few half decent laps at the Ring a few days later

However when I checked the fluid level, the fluid was badly contaminated, so I flushed it and hoped that the was just a result of the initial wear in

On my next trip to Germany for DN24 the clutch started slipping when in gear, not just in fifth, but in any gear, eventually the car simply refused to go up any incline, I couldn't even coax it the last ten miles to my hotel

Once the car was repatriated I found that the seals had become swollen, left debris on the Teflon surfaces and simply stopped the clutch from moving freely enough to fully engage

I fitted a new slave a couple of weeks ago and the clutch action was perfect again, until the morning when it wasn't

If the pressure plate spring has failed, how can it fix itself after being left idle for a few days?

[ukkid35]

gazza285 said:

Master cylinder not quite fully retracting perhaps?

So I'll try replacing the clutch master in that case

But how do I test it?

The failure only became apparent after several hours use, so I could end up stranded again - not ideal

[stevieturbo]

ukkid35 said:

I found that the seals had become swollen, left debris on the Teflon surfaces and simply stopped the clutch from moving freely enough to fully engage

I fitted a new slave a couple of weeks ago and the clutch action was perfect again, until the morning when it wasn't

If the pressure plate spring has failed, how can it fix itself after being left idle for a few days?

That is not normal. I would be wanting to know why the seals swell up. What fluid are you using ?

[gazza285]

ukkid35 said:

gazza285 said:

Master cylinder not quite fully retracting perhaps?

So I'll try replacing the clutch master in that case

But how do I test it?

The failure only became apparent after several hours use, so I could end up stranded again - not ideal

It might be the pedal that is stopping the master cylinder retracting though.

[GreenV8S]

I've seen those symptoms on brakes a couple of times. One was a worn+dirty m/c which didn't allow the piston to retract far enough to open the circuit to the reservoir. The other had the pushrod adjusted too tight with the same effect.

When the circuit is sealed, any expansion of the fluid causes pressure to be applied at the slave. Obviously the fluid will expand with heat. This will give the symptoms you found on the clutch. On brakes, it meant the car quickly because undriveable.since dragging brakes heated the fluid very quickly.

As a workaround, it may be possible to release the pressure by side-stepping the pedal so that mechanical inertia works in your favor. The solution is to make sure the pushrod has a small amount of slack at rest - if you still get the problem, service the m/c.

Edited by GreenV8S on Sunday 18th October 17:27

[ukkid35]

stevieturbo said:

That is not normal. I would be wanting to know why the seals swell up. What fluid are you using ?

No change from what I've been using for years - ATE Typ200

[ukkid35]

I've removed the clutch pack, but can't see or smell any obvious signs of damage

I am going to swap the pressure plate with a spare that I have, in case the diaphragm spring is heat damaged

I don't want to have to replace the entire clutch pack as that is £800

But is there anything I should try to test or inspect more carefully?

[ukkid35]

Also replaced the clutch master as advised, seems fine driving round the block

Unlikely to be able to test the car on track for several months, oh well