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MG MGB Technical - Brake Master Cylinder

I was told that my master cylinder was out as I had no brake reaction when pressed. Brake lines were bled. I pulled the cylinder out but the reaction seems good; squirts fluid when pressed. What are the signs of a need for a re-build? I am stumped.
Thanks, John
JW Colson

Generally the pedal sinking to the floor without fluid lose or fluid lose from the cylinder. Also check the whole system for leaks. Bob Thompson/International Auto Service
RHT Thompson

If there is no external leaks, and the brakes are bled, the internal seal on the cylinder piston may be junk. This would show as a progressively falling pedal when the brakes are pressed, but would not show a defect when there is no opposing pressure (as your bench test is demonstrating.)

This condition requires replacement of the seals on the master cylinder piston
greg fast

One sure sign of a bad internal seal is when stopped leave pressure on the pedal and it will slowly sink to the floor.
Mike MaGee

I don't have personal experience, but recall reading in the past, that it is hard to "rebuild" a MGB master cylinder and get clean honed surface that will reliably seal. New or factory rebuilt is apparently the better choice, rather than honing and putting a "kit" in.
Barry Parkinson


I've just completed a master cylinder rebuild with virtually no experience with precision work. It was a frustrating process, gave up once and put the car back together, then tried a second time a few days later. Original workshop manual was a life saver, but often not detailed enough.

In the end, I scarred the piston pretty badly, but figured I'd keep working. Sanded it fairly smooth, put the cylinder back together and bled the brake system good.

Amazingly, the car now stops with pretty good pedal. Getting the clutch bled was a bear, but I finally have the car on the road for the first time in six year.

Good luck, and don't quit a moment before the miracle happens.

Mike Kaylor

This thread was discussed between 01/08/2005 and 09/08/2005

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