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MG MGB Technical - locking rear brakes!

Hopefully a quick problem?

I have a 78 mgb roadster, one of the rear cylinders was leaking, so I ordered a new pair thinking if I change one, the other will need doing soon.

I changed the one that was leaking, bled the brakes etc, all was well except that when I drive the car and brake hard, the one rear wheel locks up with very little effort.

I rebled the brakes and it still did it, so I found the old one out of the dustbin and compared it with the other "new" one that I have and find that the new one is a fair bit smaller than the older one. Its not as small as a midget one though.

I guess that this is why the wheel locks as this will exert more pressure than the other? I guess that I may have been supplied GT cylinders?

But what now, replace the other old one with the new one so that I have a matching pair on the rear axle? or re-order a new pair of roadster cylinders?

Anyone else had this problem?

Thanks
John

John Collins

A smaller slave cylinder will produce less brake shoe pressure. Even with the difference between the larger GT size cylinders and the roadster cylinders you shouldn't be getting lock up under ordinary driving.

If you didn't replace the shoes, the leaked fluid probably affected them. Usually wet(brake fluid) brakes are either not braking much at all or locking up, with little modulation between the two states.

Barry
Barry Parkinson

Also, The GT and rodster cylinders are not interchangable. The mounting is different. I would follow Barry's advice and check for contamination on the brake shoes. Ray
RAY

And of course, run the same size cylinders on both wheels.
Tom

Ray
In my 67 gt I'm running the late rear cylinders which are a little larger. I figure that they won't come near locking up given the high cf the modern front pads.

I've done some unplanned emergency stops and no sign of the rear even getting a bit twitchy.

My guess is that you could run large diameter GT cylinders in the rear of a roadster with no problem if you had modern hi cf pads in front. It isn't hard to adapt the GT rear cylinders.

Barry
Barry Parkinson

You definitely need to investigate the difference in size in cylinders on the car now. You may have been sold a completely incorrect item, or maybe your car has been modified by a PO and the backplates drilled to accept the 'other' cylinder and the original hole not welded up. But if what is on now *is* smaller than the old one as you indicate that will give *less* braking effort that side as has been said. You may need to replace the shoes both sides, and clean the drums somehow.
Paul Hunt 2

This thread was discussed between 12/11/2006 and 13/11/2006

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