MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Weird Clutch hydraulic problem

Haven't been able to find this specific problem in the archives. I recently had to replace my clutch slave cylinder as the old one was leaking. I had the usual difficult time bleeding the unit. I tried my Ezeebleed with 10 PSI in the spare tire, attached to the master cylinder with no success, then tried bleeding from the slave as suggested in the archives, again with no success. Tried the standard pedal pump/bleed screw loosen/tighten method, couldn't even get fluid to move at all. All this using up alot of expensive DOT 5 brake fluid.
Finally reattached the ezeebleed at the master, and after running alot of brake fluid through, finally got what seemed like success (saw a large air bubble come out of the slave). Full half-inch of travel at the slave, but still the pedal felt different than before. Test drove, worked great.

Now after a couple of drives in the car, I am finding that the clutch engages very low on the pedal when the car is cold, and reverse grinds just alittle but can still be selected. As the car warms up, the pedal feels just like it used to, and reverse does not grind.

Is there still air in the system, that dissolves into fluid when it is warm?

I have used DOT 5 for 7 yrs now with no other problems, in both brakes and clutch, so I don't suspect that is the culprit.

Thanks in advance for any insight you can give.

Erick
Erick Vesterback

Erick. Sometimes air remains trapped in the pipe where it exits the master cylinder (ie where the pipe goes up in a "U" before it then goes down to the slave) I suggest that 10p.s.i is not enough to give a fast and thorough flow to dislodge this air. I use up to 30 psi. It also helps if you can hold the slave piston as far down the bore as possible to reduce the space there for air to be trapped. DOT 5 is not the problem. In extreme cases it can be helpful to loosen the banjo bolt on the master cyl and push the pipe as near to horizontal as possible Jim
j soutar

Have someone hold the pedal to the floor, then open the bleeder. The sudden rush of fluid tends to carry those last hard to get bubbles out.
John H

Erick,

I had my doubts about this method myself, until I tried it. I have used mity-vac, two person bleeding etc. It is called gravity. Fill the master open the bleeder a crack and let gravity take over. I think I left it for about 4 hours, but some say over night. Pedal was perfect.

Shareef
Shareef Hassan

Erick - If all else fails, bleed normally, then crawl under the car, disconnect the clutch rod from the fork and push the rod as far into the slave cylinder as possible (pushing the piston all the way back into the cylinder and expelling air trapped in the cylinder into the hydraulic line), reconnect the rod to the fork and re-bleed normally. This will often get that last bit of air out of the system. Cheers - Dave
David DuBois

I've reverse-filled/bled clutches a couple of times and had no problems at all. The first time using an Eezi-Bleed on very low pressure connected to the slave bleed nipple, the second time by interconnecting the right-hand caliper and clutch slave nipples and using the brake pedal gently to fill and bleed the clutch in one go.

If it worked OK for a while, but now seems to have air in it again (has the slave piston travel reduced?) I'd be concerned that there was a leak or it was sucking in air.

You should need to disconnect the slave pivot to push the piston all the way back into the cylinder, but an extension to this method *is* to remove the pivot, then use the pedal to push the piston out as far as you dare, leave it a while for any bubbles to drift to the top, and *then* push the piston all the way back into the cylinder. This should push a whole lot of fluid back taking any air with it.

I've heard of the 'crack it open and let it drip overnight' method several times now, but just don't see how it works. Where does the air go? Up (to get trapped in the U-bend?) or down (against gravity?)?
Paul Hunt 2

Thanks all for your help so far. I think the reason my reverse bleed attempt may have failed was that there was fluid leaking by the threads of the bleed screw. When undoing it, I noticed that the screw is kind of slack in the threads. Would some plumbers tape (that white non-sticky stuff) be safe to use there?

The other problem I had was that the master wanted to overflow all the time, and by the time I would see the fluid in the reservoir then run around and disconnect the spare tire, it was overflowing. I didn't have a helper at the time.

If the air is trapped in the bend near the master, I am tempted to try the reverse bleed again, if I can seal the bleed screw well enough.
Erick Vesterback

After replacing my slave, I encountered the same problem. By removing the slave from the trans and using a c clamp to force the slave piston all the way in until is bottoms out, you now can use the EZ Bleed with success. One other pointer, make sure that the the slave bleed screw is at the highest point of the cylinder. Ray
RAY

Maybe you will have to replace the slave. Or just the bleed screw. But teflon tape isn't the way to go here. The seal is at the bottom of the bleeder, and there is obviously a hole through that, so it would still leak, and probably not bleed properly.
Tom

When pressure bleeding leaks from the threads shouldn't make a difference - other than making a mess - as it is all fluid coming out. It would make a difference using vacuum bleeding, as it will be sucking air up the threads and closer to the cylinder, which is not where you want it to be. And yes, when reverse bleeding (as opposed to filling) it is necessary to empty some out of the clutch master first, as it makes an even bigger mess :o( Less so reverse filling, as one just keeps an eye on the level in the master and stops as soon as one can see fluid above the top of the cylinder casting. And while I'm at it when using the brake pedal for reverse filling/bleeding the clutch you need to make sure the level on that doesn't go down too far or you will have to bleed those as well!
Paul Hunt 2

Thanks to all who responded to my original post - I have the problem solved now it seems. I re-connected the Eezibleed to the master, pumped my spare up to 20 PSI, and then ran a half-pint of fluid through the slave bleed nipple with the nipple oriented up and slave piston pushed all the way in with a clamp. I saw no air come out of the nipple, but I could tell immediately that the pedal felt more normal. A test drive showed that the clutch is working perfectly now. Its been a couple of days now, and it still seems perfect.
My conclusion is that after repeated transferring of fluid from catch jar back to the Eezibleed container when I initially bled the system, air became dissolved in the fluid. The fluid I used this last time had been sitting still for days, and when I poured it into the Eezibleed container, I poured it very slowly, with the container tilted to minimize the amount of air that would get absorbed by the agitation of the fluid.
It also may have been that air was trapped near the master fitting where the pipe goes up then down again, and that 20 PSI pushed the air out better than 10 PSI did, but I would have thought that I would have seen the air come out of the slave.
Anyway, thanks to all who helped!

Erick
Erick Vesterback

This thread was discussed between 28/12/2007 and 02/01/2008

MG MGB Technical index

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