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MG MGB Technical - brake servo

Hello, I've just come across and joined this site in the hope someone can help.

I have a 1974 mgb gt and the brakes have always been a problem, it first started when I got the car and everytime I started the car the brake fluid would get sucked through the inlet manifold and out the exhaust, I replaced the remote brake servo and now the fluid stays in the system it takes a couple of pumps to get the brakes to work.

The brakes have been bled many times with no joy.

When looking at brake servos there were two different sizes to choose from 5/8 and 7/8, are these units interchangable? if they are then could the master cylinder be faulty? As I said it takes two pumps and the brakes hold really well, drive off again and it takes another two pumps to stop.

Help please it's driving me nuts

Many thanks Mark
M Pearson

Is it still leaking fluid ? Did you rebuild it, or buy a new servo? Have you done any other work to the brakes, is the level in the master cylinder still holding or loosing fluid? Did you check the rear brake cylinders for weeping ? Have you inspected the rubber brake lines ? Condition of the rubber brake lines ? Calipers ? Did you turn the drum and rotors or have u bought them new ? What kind of brake fluid are u using ?

Have you checked to see if your rear brakes are adjusting properly, being out of adjustment can have a lot of impact on your pedal travel.
John H

Both my MGBs are as you descibe with 'normal' bleeding and that includes continuous pressure bleeding e.g. from a gunson's EeziBleed as well as from the pedal. After normal bleeding I always have to get someone to stand on the pedal while I rapidly open and shut each caliper nipple, and that always gets an extra lump of air out. After that they are fine.

I'm only aware of one remote servo, what does the 5/8 and 7/8 refer to? Port sizes? Model numbers?
Paul Hunt 2

Seems to be a leaky master cylinder. When the rubber seals on the pistons become old, this sometimes happens and the mc has to be removed, dismanteled and new seals from an overhault kit fitted. If there are no scratches inside the bore of the mc, it should be O.K. after this rebuilt, otherwise, if scratched, it must be renewed.

The problem as it is discribed is a leaky piston rubber seal on the rear most piston inside the mc. Brake fluid is forced into the servo every time presure is built up or during long rests. The vacuum of the engine then delivers the brake fluid to the inlet system where it is mixed with the gas and burned, just helping to clean out the carbon on top of the pistons.

Do not wait for a self curing of this seriuous fault, fix it as soon as possible!

Safety Fast!


A leaky master seal will cause a slowly sinking pedal (and quite possibly dripping off the pedal), it's air in the lines that causes a long or soft pedal initially that 'pumps up'.
Paul Hunt 2

This thread was discussed between 12/06/2007 and 13/06/2007

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