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MG MGB Technical - Master cylinder leak

I've been waiting for a back-ordered brake master cylinder which arrived the last day of trading before Christmas break.
I've just installed it and after a few hours scratching my head trying to bleed the system, I discovered I have a leak coming out of the pressure differential warning switch (I think that's the proper name).
The master cylinder is a dual-line with the rectangular plastic tank, by the way. 3 ports on the side and one underneath where this plastic switch is/was screwed in.
No chance of getting a new switch until after the holiday break and I was itching to take it out on Christmas Day (thus the last minute rush to get it fixed).
I have a bolt with the correct thread and length, plus an assortment of copper and stainless steel washers. Is it okay to plug the outlet temporarily and will it matter which washers I use with brake fluid coming in contact?
Any help appreciate.
D O'Brien

If you want to disable the imbalance warning then that is your decision. Plenty of MGBs still exist with only single circuit brakes so it is no big deal, it was only a regulatory requirement. Having said that, personally I hate things that don't work. I can't see either copper or stainless washers having a problem with brake fluid, but you may need fibre or rubber to seal the faces, and it may still run down the threads.
PaulH Solihull

D. A copper washer, especially if it has been annealed, makes a better sealing device than steel. And, yes, I have used a copper washer for the purpose you describe. But, that was on an old master cylinder which had been rebuilt, three times, by a local MG Shoppe and they had never bothered to rebuild the pressure differential side. Only the main bore showed new seals when I tore the thing apart after replacing it with a new master cylinder.

If, with the twin reservoir system having the balance switch in the left hand side of the assembly, you have a leak from the hole for the balance warning switch, the problem is bad/damaged rubber O rings in the left hand bore which contains the hole for the switch and the balance rod. If it is a new or rebuilt unit, it should be exchanged for one which works properly. If it is a unit that has been in use for a while, it needs to have the warning system internals taken out, the inside of both bores cleaned and inspected, then the entire system properly rebuilt.

I would hate to invalidate my warranty, on a new master cylinder or other part, by doing something like you are discussing. So, if you decide to do it, make sure you only do it for "test and diagnosis purposes" (which may contain a test drive) and take the improperly manufactured part/poorly rebuilt part back to the retail suppler and request one in proper working condition.

Les
Les Bengtson

This thread was discussed on 23/12/2011

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