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MG MGB Technical - Clutch Problem

My car is a midget but I have been helping my brother rebuild his rubber bumper MGB. We are having problems with the clutch. The internals are all new as is the master cylinder, slave cylinder and flexi pipe. After bleeding with my vac kit the clutch works fine but reverse to the bottom of the drive and the gears cannot be selected. I have dismantled both master and slave cylinders and both are fine. Since the system works after bleeding I recon the clutch itself is OK and the fault must lie with the hydraulics. Can anyone advise please.

Jan T
J Targosz

If it's like an MGB then with the clutch pedal fully down if you select reverse and it grinds then the clutch is dragging, which is often air in the hydraulics.

But if you can select reverse without grinding, and the biting point in all gears feels 'normal', then failure to select any gear is more likely to be a gearbox problem,

But that pre-supposes that reverse in the Midget gearbox doesn't have synchromesh, which the MGB hasn't. On forward gears the synchromesh prevents grinding, it just won't go in.

The other thing is that I have never had a gearbox yet, in any marque or model, where you could guarantee it would never baulk when selecting a gear, BL gearboxes worse than most. Sometime trying to select 1st I have to nudge it towards 2nd before it will go in, and to select reverse sometimes nudge it towards 1st and if that doesn't work towards 2nd. When all fails as it sometimes does, it's clutch up then back down and try again.
paulh4

Jan
You mention a new slave cylinder-
Is the bleeder fitted correctly into the top hole and hose below-
Sometimes the new cylinders come packaged with the bleeder in the wrong hole--just so they'll fit the packaging

willy

3 pics here

#1 -as packaged INCORRECTLY with the bleeder in the end
#2 -dissected view showing the bleeder port at the top
#3 -fitted up to the car correctly





William Revit

#3 - How it should look on the car--


William Revit

If the hydraulics are OK, one cause of clutch disengaging initially but then failing to do so after driving can be the thrust pad on the diaphragm being out of true. The shimmy gradually pushes the release bearing away, so some of the pedal stroke is used in getting it back there. We have found new units are not immune to this fault and check each one after bolting up.

Best check for hydraulics is there should be 11mm stroke at the slave. Less than 10 is a problem.
We have also found some aftermarket slaves lack the support under the seal and can suck air back past the seal.
Paul Walbran

Check the clevis pin at the master cylinder for wear. Be sure to grease the pin as this will stop wear in its tracks. If the pin is badly worn, so it the fork and as they are "wear parts", they should be replaced together. Also check the slave cylinder rod and clevis pins for wear. The pivot arm on clutch fork also wears. Add the wear on these parts up and the geometry is such that it reduces the throw considerably.

Be sure to leave the bled system overnight and do a fresh pull in the morning. Inevitably you will get a big bubble or two.
Glenn Mallory

Whilst wear in the mechanical linkages at the pedal end does contribute to a low biting point wear at the slave end doesn't as the slave piston just mover further out to take it up. That's what makes this clutch system self-adjusting, and is essential given how much wear can occur on a graphite release bearing yet still functional normally.

Leaving the pedal fully depressed overnight then slowly released next morning is a great help in fully bleeding the system.
paulh4

This thread was discussed between 08/10/2023 and 24/10/2023

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