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MG MGB Technical - More clutch talk

I've asked a few questions regarding clutch replacement over the past week and have a pretty good idea how to proceed........if I have to.

The clutch was slipping in 3rd and 4th, but isn't now.

A bit of backgound: early this summer I rebuilt the clutch master and slave cylinders. I did this along with a new hose to the slave and fresh Dot4 fluid after flushing the newly rebuilt system.

I thought I did a pretty good job as the clutch worked quite well after the rebuild.

However, after about 3 months I discovered the rebuilt master cylinder had been slowly leaking out the push rod boot, and the clutch pedal went to the floor. I refilled it with the intent of bleeding the system again but didn't because the clutch worked as soon as I had topped up the resevoir.

I decided to keep an eye on this problem and continued to drive. Don't know why I didn't think of all this when the clutch started to slip....guess it just didn't seem logical that it would be part of the problem. No pedal means the clutch remains engaged....seemingly the opposite of a slipping problem.

Then I decided I'd better solve the leak issue before proceeding with the clutch replacement. Yesterday I put in a new master cylinder and lo and behold, no more clutch slippage.

Is it possible that somehow the leaking rebuilt master cylinder put pressure on the slave cylinder, thus preventing it from fully disengaging.....sort of like driving with your foot resting on the clutch pedal? Could a leak out the push rod end be consistent with some sort of failure to relieve pressure internally?

Also, there does not seem to be any adjustment in the hydraulic system. In trying to maintain a bit of play in the slave ram to gearbox lever connection all I could do was take advantage of the slightly oversize bolt holes in the slave cylinder when bolting it in place. I was just trying to make sure there was no pressure on the gearbox lever, and subsequently, the clutch.

By the way, the slave cylinder was completely dry...the rebuild apparantly being successful there.

Thanks,
BH

BH Davis

Sounds like the master was holding pressure on the slave. I think the leak was a separate issue.
Glad you are road worthy again.

Steve
Steve

The hydraulic system is self-compensating for wear in the linkages at the slave end, the driven plate, friction plate and flywheel. It is the spring in the cover plate that pushes the slave piston back into the cylinder, which pushes the fluid back into the master reservoir, and so the resting place of the slave piston gradually gets further and further out of the cylinder as the clutch etc. wears. Replace the clutch, and the piston will then sit further into the cylinder again. There is another spring in the system and that inside the slave cylinder, on the hydraulic side, and this is tending to push the piston out of the cylinder all the time. With even very slight end-float of the crankshaft the piston would be pushed back into the cylinder and *create* play in the system, which would be apparent as a low releasing/biting point the next time you used the clutch, but would be very inconsistent. This spring makes the releasing/biting point consistent over the life of the clutch. However this doesn't mean the releasing/biting point remains the same over the life of the clutch, because as the clutch wears the spring pressure reduces, which means it disengages with less pressure on the clutch pedal, which translates as the release/biting point gradually getting higher.

This means that if the clutch is slipping because fluid pressure is being maintained for some reason, the release/biting point will be very obviously high, compared to when it isn't slipping. One cause of trapped pressure is a delaminating flex hose acting as a check valve, and this may come and go. The flex hose should always be replaced with the slave cylinder (unless it is almost new anyway) as it is much less work to do both together than one at a time. Another cause of trapped pressure is if something is stopping the master piston coming all the way back to clear the port between the cylinder and the reservoir. This has to be clear whenever the clutch is released so fluid expansion and contraction can take place with temperature changes. If it is blocked then when the fluid expands it will take pressure off the clutch and cause slipping, and when the fluid contracts (or if the flex hose is delaminating) it is likely to suck in air past the seals.
Paul Hunt 2

Paul,

Good stuff.......thanks! When I wrote the post I'd forgotten that the slave cylinder piston was completely free floating with no snap ring outward travel limitation.

Turns out that I was wrong about the slippage though....it still is slipping. I'm afraid it is time to pull the engine and do the job. I've gone over pretty much everything in the system now....including a new slave cylinder hose when I did the first rebuild this past spring.

Thanks for the great explanation.
BH
BH Davis

This thread was discussed between 20/10/2007 and 23/10/2007

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