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MG MGB Technical - Clutch slave piston

Can anyone tell me how much the clutch slave piston needs to move to activate the clutch. I've got to the stage of filling the system with fluid and just wanted to check the movement was sufficient before engine start - the first for over 5 years!
Richard Coombs

Richard. The times I have measured the movement of the pushrod, it has been about 3/8" on several cars. Others have claimed they measured about 1/2" of movement on their cars, but I have never seen this much movement myself.

Les Bengtson

Last time I checked the movement was in the region of 9/16" to 5/8". Any less than this and the clutch just will not clear.
Iain MacKintosh

Interesting. I measured just over 5mm so I guess I've still got some air in there somewhere.
Richard Coombs

MGB clutches are notoriously difficult to bleed top-down, even with a continual flow system like the EeziBleed. So much so I didn't even try the first time I had to use it, and used reverse-filling/bleeding instead. Initially from an EeziBleed on very low pressure connected to the slave nipple, subsequently I linked the RH caliper to the slave (same sized nipple) and had someone operate the brake pedal *slowly and gently!* while I opened and closed the brake nipple at the appropriate times. Got full travel (1/2" IME) straight away each time. If your master is full you will have to drain some out first to stop it overflowing.
PaulH Solihull

Paul I like the sound of the brake method.

I have been using bottom fill with an EeziBleed and a tyre with about 8 to 10 psi in it. The problem has been you have to wait untill all the air has been pushed out of the plastic tube before pressing it onto the slave nipple without losing to much fluid. The possibility of fitting a valve/tap in either the air or fluid tubes had crossed my mind to make it more controllable.

Operating from the brake would give that much more control.
David Witham

"The problem has been you have to wait untill all the air has been pushed out of the plastic tube before pressing it onto the slave nipple without losing to much fluid."

Shouldn't need to do that if there is plenty of space in the reservoir.
PaulH Solihull

I bench bled my slave cylinder several months ago and put it on the shelf. Last night we bled the clutch line the old fashioned way with two people, including holding the cylinder upsidedown. I then assembled the slave cylinder to the bell housing and lever, and we only had about 1/4 inch of movement, all of which was very close to the bottom of the pedal movement. I was hoping the cylinder would be like a disc brake and simply push the cylinder out to the needed amount before returning, but it didn't work that way.

I went back under the car with my rubber hose and my bleed jar and filled the rubber hose with a couple of pushes on the brake pedal from the front disc, and then connected it to the slave cylinder. Diane gave two pushes on the brake and I closed the slave cylinder bleeder after each application.

We had immediate success, and now have a much firmer pedal and about 5/8 inch of movement. I placed the gearbox in gear and checked that the release of the clutch was well off the floor.

The method really works.
Dave Braun

Richard I put a new Lockheed slave cylinder on my B today and it has around 5/8" movement, maybe more. I bled it the conventional way with my wife pushing the pedal. The secret is to prevent the piston from moving by using a clamp so that the piston is pushed in as far as it will go.

Push the pedal, open the bleed, close the bleed and release the pedal. Repeat many times. I found that after 4 pedal depressions you needed to top up the master cylinder.

It didn't take too long and the clutch works great.

Andy Preston

Thanks Andy. I've seen this technique discussed on Paul Hunt's website where he gives useful advice on precautions etc. I think I'm going to try the brake connection route as I also want to experiment with the reverse technique i.e. use the clutch to back-fill the brakes.
Richard Coombs

"use the clutch to back-fill the brakes"

Did you mean that? Or the other way round? The brake system is easier to bleed than the clutch, but has also needed to be done differently on the several MGBs I have worked on - low-pressure for filling, which leaves it with a spongy pedal, high pressure to blast the last bits of air out. It would be interesting to see if reverse bleeding the brakes avoided that high-pressure stage, but would need several drain/bleed forward/drain/bleed reverse/drain/bled forward/drain/bleed reverse iterations to check that you got the spongy pedal after each forward bleed but not after each reverse bleed. I think I'll leave that to you :o)
PaulH Solihull

Yes, at the time of writing, I was thinking of using the clutch to backfill the brakes. The reason was because I had to undo the banjo at the back of the brake master cylinder to cure a leak. So I reasoned that rather than push the air now in the top inverted u-bend of the brake pipe all the way to the brake cylinder I would use the clutch to push the air the other, shorter, way. I've changed my mind as I am not at all sure that with the clutch not operating correctly because of trapped air the whole scheme will end up being somewhat circular i.e. air from clutch to brakes then air from brakes to clutch. I will try the brake to clutch method after I've bled the brakes.
Richard Coombs

This thread was discussed between 31/03/2012 and 16/04/2012

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.