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MG MGB Technical - Brake Master Cylinder

I have a 1974 MGBT with a single brake system.Due to brake fluid leaking from the piston I decided to replace the master cylinder rubbers.I was unable to remove the lower nut on the cylinder due to it being in a very diffcult position.I removed the pin from the operating leaver pulled back the rubber boot removed the circlip the piston was removed new rubbers fitted time taken 10 mins.
M Scull

When MCs leak, the bore is usually scored and / or pitted. Just replacing the rubbers doesn't help for long.

I had a bit of a nightmare when my MC, in my 66 roadster, started leaking.I eventually got that bottom bolt out, can't remember how. The bore was pretty badly pitted.
Anyway the decision was whether to have it resleeved or buy a new one. I thought I'd go for originality and have it resleeved, at about twice the price. When it came back it had been bench bled and was half filled with brake fluid. Messy re-fitting it.
I should have realised that there was a problem when the brake pedal came up higher than before.
Anyway did the bleed ing trick and on my first drive the pedal went lower and lower, before starting to slow the car. Very worrying.
Checked everything and no leaks anywhere, only brake fluid coming out the top of the MC reservoir.
I then gave up originality and bought a new MC and swapped the two over. Brakes OK now.

Out of curiosity I went to dismantle the old MC to see what the problem was. When I took the rubber boot off the piston et al popped straight out. What had happened was that when they bored out the cylinder to fit the sleeve they didn't cut the circlip groove any deeper, so it didn't have any purchase and just fell out. I was lucky that it was a gradual failure and not a sudden one.
Went back to the brake shop, complained bitterly and got my money back.

A photo of the original bore is attached showing the pitting.


Herb Adler

M, hats off... necessity is the mother of invention and can I be so lucky (or have fingers with that much dexterity) please. I sincerely hope you get away with it. As Herb says "the bore is usually scored" but not always...give a a few weeks and check again.

Best of
M McAndrew

I've changed a brake cylinder and a V8 clutch slave this year and both had failed because of erosion of the bore, leaving cavities that the seal could no longer cope with. Part of the problem seems to be that the seals are only fractionally bigger than the bore, so there is little lee-way. That may help with assembly - both pistons certainly push in very easily whereas I remember struggling to get new brake slave seals back into the cylinders many years ago. As I say it is many years ago but I do seem to remember the old seals had developed a curved lip i.e. from wear, whereas the new ones have a sharp lip. The seals on both this years still had a sharp lip, and both had been changed before in my ownership.
PaulH Solihull

I did check the bore as much as possible and it seem to be in good order I am keeping my eye on the fluid level and to help I have made a simple dipstick out of carboard in the shape of a letter T.
M Scull

Hope you're the lucky exception, but usually new seals don't fix the problem....
If you should have to pull the m/c, and replace it, consider replacing the clutch m/c at the same time, so you won't have to do it again, as it's such a major PITA!
As mentioned, the seal kits are only slightly larger than the bore, and any wear in the bore, will allow it to leak....Of course, any corrosion in the bore, will do the same...
It's always been my contention that the replacement seals should be even larger than the originals, to allow for honing of the bores...But then, the manufacturers wouldn't be selling so many replacement master cylinders.
E.B. Wesson

This thread was discussed between 14/12/2012 and 19/12/2012

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