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MG MGB Technical - Clutch Hydraulic Problems

I rebuilt the clutch M/C on my '74B. I bench bled it and it seemed to be pushing fluid correctly. Now that I've installed it, nothing. I've pumped it 50+ times and there's no build up in pressure and no loss of fluid. Am I overlooking something obvious or do I need to pull the M/C and start over? Thanks.

Dan Hiltz

i had the same problem with an aftermoarket cylinder. The pedal didnt come back far enough to allow the pushrod to fully disengage allowing the fluid to drop.

A P New

So how did you correct it?
Dan Hiltz

Have you tried reverse bleeding --by forcing fluid from slave back to master --it works well. I use a dedicated oiler for brake fluid only to reverse bleed
Gil Price

I haven't tried reverse bleeding. That's really the purpose of my post. If the expertys here thik that the problem will correct itself once the system is under pressure, reverse bleeding is a great option. On the other hand, if I need to pull the M/C, I'd rather do it before I put more fluid in the system. Does thatmake sense?
Dan Hiltz

Sounds like you are swishing fluid backwards and forwards in the M/c. Did you replace the push rod? Does the pedal go all the way to the floor?
It's also not unknown for the lip seals to turn on assembly, assuming you assembled the right way round. No disrespect but it does happen!!
You say you bench bled it. It would probably move fluid OK without the back pressure of the the clutch, this resistance is probably just pushing fluid past the M/c piston.
Allan Reeling

Thanks Allan. Right now there is no back pressure; the bleed valve is open. I didn't replace the push rod, just the piston. Everything (pedal, linkage, pedal box) is the same as when I removed except for the M/C internals, and they seemed to work during the bench bleed.
Dan Hiltz

Reverse bleeding will tell you if the piston is coming back far enough to clear the bypass hole or not, if it isn't you won't get any fluid flowing back into the master. Pedal bleeding is probably the worst way to bleed the clutch, even if you have someone opening and closing the bleed nipple at the appropriate times. Even continuous flow from a gunson's often doesn't work. You certainly want to try reverse bleeding before removing the master again. The easiest way is to remove some fluid from the clutch master to make space, link the right front caliper and clutch nipples (they are the same size), open the clutch slave, and while some applies *very gentle* pressure to the brake pedal open the brake nipple. Very slow and gentle pressure should be applied to the brake pedal or you will blow the tubing off (unless you use hose clips). When the brake pedal gets to the end of its travel close the brake nipple before letting the brake pedal back, then open the brake nipple again. Just a couple of pumps should do it, you will probably hear much gurgling from the clutch master in the process. Watch that the clutch master doesn't overflow, or the brake master get too low if for whatever reason you keep pumping. You should end up with anything from 3/8" to 5/8" of travel. Older systems seem to have the longer travel, and a heavier pedal as a result. More recent slave replacements in my experience have been lighter with less travel, but still give the correct biting point, possibly due to an oversize bore.
PaulH Solihull

OK. The reverse bleeding moved fluid from the brake M/C to the clutch M/C, and I did it until there was no air surfacing in the clutch reservoir. Still no pressure on the clutch pedal. Do I need to do a traditional bleed now? I would expect to get some pressure build up after pumping the clutch. Suggestions?
Dan Hiltz

If there is no fluid loss it sounds like the master seal is failing, maybe inserted backwards.
PaulH Solihull

After my last post, I actually got some pressure and some movement at the slave cylinder. Before I pull the M/C I'm going to use a clampe to compress the slave and see if I get better results.
Dan Hiltz

I'm having the same problem on my '80 MGB. Took it out of winter storage and lost the clutch within a few hundred yards. Found a leaking slave cylinder and replaced it, the master cylinder and the hose to the slave. I've checked all the connections for leaks numerous times, but keep getting air when I bleed it using the conventional pedal method. I've never had a problem bleeding other vehicle clutches.
I suspect build quality of the new replacement parts (Taiwan) and the bleed nipple feels awfully loose in the threads until fully tight. Has anyone else tried the methods in the threads?
J.S.V. Smith

Are you closing the nipple for each upstroke of the pedal? Even then if the seal isn't very good you could be sucking air past the slave seal instead of fresh fluid past the master seal. This is why pressure bleeding is best, and reverse bleeding best of all. The reason there are so many alternative methods quoted in threads like this is that the MGB clutch *is* a pig to bleed, so try one of the recommended alternatives.
PaulH Solihull

It took several tries but I believe it's done.
1) I bench bled the M/C before installation.
2) I followed Paul's always helpful advice and reverse bled from the right fron brake. That got more fluid into the system but it still wouldn't hold pressure.
3) I spoke to John Twist and followed his advice to tighten the banjo bolt as much as possible (since this can be accessed throught the firewall). He also suggested waiting 20-30 secodnds after each upstroke on the bleeding process to allow fluid to be drawn from the reservoir.
4) I used a C clamp to compress the slave cylinder and tried to bleed that way. Pedal was not able to be depressed at that point.
5) I followed with one more round of traditional pumping to bleed.

Seems to working now. I'll post if I encounter any more problems.
Dan Hiltz

This thread was discussed between 28/04/2012 and 02/06/2012

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