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MG MGB Technical - Clutch bleeding not working

I have a '79 MGB. The clutch hose was leaking at the slave cylinder. I changed it out and I'm having a tough time bleeding. I reverse bled the clutch system using the front brakes through the bleeder. I then sent the fluid up through the system by pushing in on the yoke, getting a satisfactory sound of bubbles in the pipes. When my assistant (wife) presses on the clutch pedal the yoke moves. This is all done while the slave cylinder is disconnected from the transmission. When I hook the slave cylinder up to the transmission pressing on the clutch has no effect. The gear shift stays in neutral and won't go into gear. If I start with the car in gear the car will start to roll with the clutch still depressed.

Any help with diagnostics will be appreciated.

Thanks,

Jim
JMcHugh

Jim, I also have a 79 MGB. I've discovered if you leave the car alone with the cap OFF the reservoir (over night), it may self correct. It's a bit of a long shot, but it has worked for me on both the brake and slave cylinders.

If that does not work, you may have to "bench bleed" the slave cylinder to get trapped air bubbles to release. This may mean you need to remove the slave cylinder from the car and actually bleed the cylinder on your work bench and then re-install it. Allow the fluid the run through the line as you are re-attaching the line to the cylinder without pumping the clutch pedal. Top off the cylinder very slowly as not to create air bubbles.

Cheers

Gary
79 MGB
gary hansen

What has often been suggested is wedging the clutch pedal down and leaving it overnight. I left mine for a couple of days and it seemed to work.
Dave O'Neill 2

Does the clutch slave have the hose / bleeder in the correct positions ---hose in the end bleeder on top
There is a tendancy for new cyls to have the bleeder in the end for packaging purpouses
If you cyl has been replaced previously it may be mixed up
William Revit

Bleed it from the slave.

Note that the flex hose should be in the back of the cylinder and the bleed valve in the side.

Loosen the bleed valve and pull the fork back into the slave to eject air & fluid. Tighten valve and release the fork - repeat until clear fluid with no air comes out.

(Willy posted while I was typing!)
Chris at Octarine Services

Thanks, I've taken the cap off for the night. If that doesn't work I'll try the bleeding through the slave. I am using the end for the supply and the side for the bleed screw.

Ill let you know how I make out.
JMcHugh

Reverse bleeding from the front caliper worked for me in the past, but more recently it's needed the pedal to be wedged down overnight, maybe more than once.

Also try clamping the push-rod and hence the piston all the way into the cylinder, with both top-down and bottom-up bleeding.

There are theories why both of those work, but leaving the cap off overnight is a weird one as the cap is (or should be) vented anyway.
paulh4

I also tie the arm/pushrod back with some strong wire, to eliminate any trapped air in the slave cylinder.


Dave O'Neill 2

Here's the results so far. Leaving the cap off overnight didn't help. I started bleeding the slave and got slight drips, not a flow. Then I pushed the clutch pedal down and got a marginally stronger flow. I left it for about two hours and when i went back to the problem there was no flow from the slave bleeder. As a matter of fact it seemed as if there was nothing in the slave cylinder. The fork seemed to go back and forth with no resistance. I have not tried tying back the fork because I'm not sure what would happen with both the clutch pedal held down and the yoke tied back.

Still no positive results.

Jim
JMcHugh

Sorry, I don’t think I made it very clear. I tie the arm back when bleeding.
Dave O'Neill 2

But if there was a significant amount of air in the system i.e. no movement at all at the slave end when putting the clutch pedal through its full travel then doing both would have a greater effect.

If there was some movement at the slave end then you wouldn't want the slave piston tied back when operating the pedal with the bleed nipple closed or you could blow the seals. Only do that with the bleed nipple open, although really the best approach is to apply some pressure to the pedal before opening the bleed nipple and then close it again before releasing the pedal. Also bleeding - both clutch and brakes - should be done with the front of the car slightly higher than the rear.

But from what you have said about the no flow, and particularly "I left it for about two hours and when i went back to the problem there was no flow from the slave bleeder" makes me think there is a blockage somewhere, or maybe the master seal has failed. Forgive me, but I assume the master cylinder still had plenty of fluid?

If you can get hold of a pressure bleeder to try i.e. EeziBleed and get a stronger flow from than you do from the pedal (it should be the other way round) then I'd say the master is faulty.
paulh4

If you have the pedal tied down as you say there will never be any flow --as soon as the pedal is moved from rest the reservoir supply gets cut off
Leave the pedal up open the bleeder and wait for the flow from the bleeder to stop bubbling , close the bleeder ---done
The more you pump the pedal and stuff around the more the fluid gets frothed up if there's some air in there
Slow and steady wins
Reverse bleeding an MGB slave from the brakes won't get the air out as the hose isn't at the top of the cylinder and there would be air above the hose port in the cylinder----It can only be blead out by the bleed nipple, that's what it's there for

willy
William Revit

The point of wedging the pedal down is that it is done overnight to allow bubbles to flow to the top so they are flushed back into the reservoir when you release the pedal next morning.

No one is saying that the pedal should be left down while actively trying to bleed the system.

paulh4

I am replacing all the flex lines in my brake system. The other day I had the experience of my front left brake staying locked up briefly after braking. On investigation I determined that the flex line was almost completely blocked thus not allowing the fluid to flow back when the pedal was released.

Could it be that the line between Jim's master cylinder and his slave cylinder is blocked? If the line is disconnected from the master, does fluid flow out of that cylinder when the pedal is pressed? If the line is disconnected at both ends, can he blow compressed air through the line?

Jud
J. K. Chapin

Jim - we have all been here in the past. Reverse bleeding is the standard solution. By chance, I found an easier way - bleed normally from the master - when there is no more air in the bleed tube but the lever still doesn't move, push the lever by hand into the slave and watch the air escape through the bleed tube - when it stops, tighten the bleed nipple and you have a working clutch.
Roger Walker

It really is a case of being aware of all the possible methods and going through them until you get a working clutch. The first time I did it reverse bleeding worked first time. More recently I've had to wedge the pedal down overnight, and having the nose of the car slightly higher than the rear helps. In theory pushing the slave piston fully into the cylinder then allowing it to refill slowly, several times, should do the same but hasn't always worked. As a last ditch wedging the pedal has always worked ... so far.

I'll echo the warning about pumping the pedal vigorously with the bleed nipple closed, having seen that as a recommendation. All it did for me was turn the fluid milky with tiny air bubbles and I had to flush the whole lot out and keep going until all the tiny bubbles had gone. Then revert to plan B ... or was it plan C?
paulh4

To be honest, I can't remember ever having trouble bleeding an MGB clutch
Just use the bleeder
Push the pedal to the floor , let the air out of the bleeder, nip the bleeder up and slowly release the pedal----slowly
Repeat until there is no air--Should only need 2-3 bleeds at the most

If you haven't got a helper
undo the bleeder, push the fork towards the slave, nip up the bleeder and release the fork--slowly, Wait a tic till the slave piston returns
Repeat , undo the bleeder, push the fork ,nip it up and return
Repeat till there's no air bubbles from the bleeder

It's not hard

Don't let the reservoir empty while bleeding

Ford Transit vans are well known for being a pain to bleed----The trick with them and something you could try on your MGB if it's being stubborn is--
Get a length of clear plastic hose, push one end onto the bleeder and the other end into the M/cyl reservoir, open the bleeder and 'slowly' push the pedal down, slowly release, slowly down, and just keep going until you have clear fluid with no bubbles in the hose, tighten the bleeder and remove the hose--finished
William Revit

Well, in the early days problems with conventional bleeding were of the most frequent complaints I was reading, which is why the first time I had to do it I didn't even attempt it and bottom-up took minutes. The problem, especially with pedal bleeding, as that you are trying to push air down quite a large bore tube, and it wants to float back up, which is why reverse bleeding works. In theory continuous bleeding from something like an EeziBleed should be better, but you have to be careful with the amount of pressure with those or the seal pops out of position and fluid goes everywhere.
paulh4

I had a very similar problem recently.
But first be sure you have the bleed valve in the right place on the slave cylinder. Very often, simply to fit more easily in a small packaging/box, the bleed valve is screwed into the input aperture and needs to be moved to the other aperture.
In my case, I initially reverse bled the clutch but I was still unable to get a firm pedal. I then purchased "eezibleed" as recommended by others here already, and the job was completed in just a few minutes.
Well worth the £20 I would say.
Graham V

Well-- No-one has quoted the workshop book which surprises me a bit -------------

It says-
Fit a clear hose to the bleeder with the other end submerged in a container into the fluid being used, fill the reservoir, open the bleeder 3/4 of a turn and slowly operate the pedal in full strokes up and down ,maintaining fluid in the reservoir until a clear flow with no bubbles is seen in the tube--tighten the bleeder on a down stroke Remove the hose and top up the fluid to the correct level-

the hose being in fluid in the container would create a resistance to air being sucked back as the pedal is lifted during pumping
William Revit

That's funny, I see two or three posts that mention that process, including your own.

The hose in fluid doesn't really create resistance, it sucks fluid up instead of air.
paulh4

>>The hose in fluid doesn't really create resistance, it sucks fluid up instead of air.<<

I'm not sure exactly what is meant by this.

I only ever use a hose with the end submerged in brake fluid at the bottom of the jar method of fluid flushing, changing and air bleeding, and usually by myself, on serval classics and some moderns.

I've never noticed fluid being sucked up instead of air - perhaps it does and I've never noticed, I'm just happy to have such simple equipment making the job very easy, steady and stress free.



Nigel Atkins

ditto -Nigel--simple works for me
William Revit

It's been awhile since I've posted. That's because I found that it was actually the slave cylinder that was bad. I replaced it and all seems well now. Thanks for all your help.

Jim
JMcHugh

Super!!! Thanks for posting the results of your dilemma.

Cheers

Gary
79 MGB
gary hansen

I have had good luck by removing the slave cylinder and letting it hang down overnight.. Hanging straight down lets the air bubbles work up to the master cylinder.
By chance did you have the hose on the top entrance.If so place it on lower and the bleeder itself on the top outlet.Sandy
Sanders

I will say this only once (and I receive no commission from the company), Eezibleed!
Graham V

He's fixed it already
William Revit

Oh yes, thanks. Guess its lucky i said it only once!
Graham V

This thread was discussed between 13/01/2020 and 24/01/2020

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