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MG MGB Technical - Clutch bleeding - engine out

I had planned on replacing the clutch hose on my '73 BGT anyway but now the engine is out, access is a lot easier so I will get on with the replacement.

Might be a dim question but can I bleed the clutch with the engine out?

As an aside the graphite clutch release bearing does not look happy - the graphite bit fell out. I take it this not how it should be? I thought it should be bonded/pinned to the carrier?


Mike
Mike Dixon

Yes, using most of the usual ways EXCEPT the pedal! That will push the piston out of the end of the cylinder. Care would also be needed with a gunson's EeziBleed not to use too high a pressure.

Screw the new hose into the slave first, with the slave attached to the bellhousing, and that will align the hose to the chassis bracket and metal pipe correctly before you tighten either of those. If you do it the other way round the slave will probably not lie flat against the bellhousing which would put a twist in the hose.
paulh4

Yes, the release bearing is toast. It should be tight in the housing. Avoid any where the carbon is retained by the pin, they fail early.
Paul Walbran

Regarding PaulH comment on first connecting the hose to the slave cylinder. I had to do it differently on my '67 B.

The hose is secured to the chassis bracket with a large 15/16" nut and a "shakeproof" washer (per workshop diagram). I was missing the washer and bought a spit ring washer. I could only get a wrench on that large nut. However, there was no room to tighten it.

Instead, I attached the hose FIRST to the slave cylinder and let it hang while SECOND I tightened the hose into the secured nut. Finally, I adjusted the rotational fit of the hanging slave cylinder to the bell housing nut holes. Allowing for a final tightening with minimal hose twist. Whatever works!
ESenkowsky

If you mark the rotational position of the hose with respect to the chassis bracket with the slave loosely attached to the bell-housing, then yes you can fully tighten the hose to the pipe nut and chassis bracket with the slave hanging down. If you don't do that, or attach the slave to the bell-housing before tightening those nuts, then you can end up with a major twist in the hose.

The critical thing is to tighten the hose into the slave before anything else.
paulh4

Finally got the engine back in today - no clutch. After a bit of a panic, it seems the clutch works fine mechanically just not on hydraulics. There is very little pedal pressure and it would not pump up.

When the engine was out I used the method of cross-connect the nipple of the right front disc brake caliper and clutch slave cylinder bleed nipples.

So today, after a lot of faffing out, re-bleeding etc, it seems the master cylinder is only shifting about 5 ml of fluid when the pedal is depressed. I wonder if I have damaged the ancient seals in it by pumping fluid into it the wrong way? I may of course have totally misunderstood this method of bleeding.

Whatever looks like time for a new master cylinder or a rebuild. Everything else is new - copper pipe , hydraulic pipe and slave cylinder. I am favouring a totally new one

Any thoughts?

Mike Dixon

When I've taken engine and gearbox out I've left the slave dangling on the hose and it has been fine when put back.

If you opened the hydraulic circuit it you probably have air in there and may have to go through the many different options for bleeding. But if you didn't open the circuit it could mean seals are damaged, you have air in there anyway, and may need to do something about seals before you will fully bleed it.

I've no idea how much fluid the master should displace, although it could be calculated from the both and push-rod movement, how did you measure it out of interest?

Reverse bleeding shouldn't have affected the seals if the pedal was fully released and the bypass port to the reservoir was open. If fluid passed easily from the brake caliper into the clutch reservoir then it was clear, but if not then it wasn't which would mean the bypass port isn't clear and that would prevent bleeding.

I have changed masters in the past but because the bores were always sound last time I just replaced the seals and it has been fine. There are two types of innards and you have to get the right seal kit, they vary, and supplier info is sometimes confusing if not incorrect. One of the kits allows you to convert from cup-type to ring-type main seal and is preferable. More info here http://www.mgb-stuff.org.uk/clutchtext.htm#cmkit
paulh4

Thanks. I have gone for it and taken the master cylinder out, new one should be here tomorrow.

Great fun - the problem was not the classic lower nut - I got at that, as you suggested on another thread from the side. But I am not looking forward to getting it back in though, I think I will try the superglue approach if I cannot get it heli-coiled.

The problem was the brake pipe ran over the top of the clutch cylinder on it's way to the servo and the only way I could get the master cylinder out was by disconnecting that pipe - which did not give up easily. I just hope it seals up again OK

Having had a quick look at the master cylinder it does not look very good, hopefully the new one will sort the problem. if not I need to improve my bleeding technique.


We measured the displacement by opening the valve on the slave and collecting a sample which we measured in a syringe - no idea if that was a thing to do but it made us feel at least we were trying to work our what was going on. We tries all sorts but no pedal pressure at all.

Thanks for the link to the seals - I think I may get a set, worth having on the shelf for the price and to take on longer journeys. Someone I know had a TR6 slave cylinder fail on a rally 14 days after it had been fitted.
Mike Dixon

Decades ago the recommendation was to get your car serviced before a big journey. I never did then, preferring to get a few miles under its belt to confirm everything was OK even though I work on my cars myself, and I would never do it now with replacement parts given the number of premature failures one reads of. Of course leave it too long and they will fail anyway these days! I only replaced the roadster bottom hose it came with after 20 years in my ownership, it had been off and back on a couple of times as it was still perfectly smooth and pliable and still is so have it a a running spare.
paulh4

I agree, I like a few miles after any service/work before setting off in earnest.

Just to close this one off and add ( if that is possible ) to the info on clutch bleeding:

- I got the lower nut back on by super-gluing the nut to the master cylinder. It held more than long enough to get the bolt started
- I managed to get a 1/4 in drive socked on a stubby ratchet handle on the nut from the side so tightening up was not an issue.
- we (under direction of my local professional mechanic on his way home) bled the clutch by having the bleed fluid bottle above the slave cylinder and pumped the pedal very very slowly by hand both up and down, closing off the bleed nipple for the up stroke

Mike Dixon

This thread was discussed between 21/09/2021 and 12/02/2022

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