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Brake master cylinder


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Checked the GT6 after leaving it for about 6 weeks, and found the brake fluid level had dropped in the master cylinder. The master cylinder seemed to have sticky yellow residue all over the outside, and there is fluid on the bulkhead, but the pipe union does not appear to have been leaking. Can brake fluid seep out of a porous master cylinder? I have had this problem in past winters, and with previous master cylinders. Any thoughts?

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Never come across a pourous cylinder, unusual but all is possible


do you use dot4 or ......a silicon fluid

One easy test ...dot 4 washes off with water


but most spills are from the cap seal

its breather hole,

the pipe union

or seal falure out into the cabin footwell


So no special clues on this one Pete

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Had something similar when I fitted a new master cylinder.  Leak initially stemmed by reducing the level of fluid in the cylinder to about halfway and then when driving the car no leak thus pointing to the cap not sealing properly.  Tried one new cap with no success, tried again with another and sorted; on investigation the 'rubber' seal was a slightly different thickness and consistency (if that is the right word).  I would suspect the leak occurred the last time you used it and check the seal first, and mine uses DOT 4 which also left a similar yellowish residue on the aluminium.

Hope that helps.


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Thanks for the replies. Am using Dot 4. I fill the master cylinder to the line of the machined flange inside, about 1 cm down from the top.

Will have a good look at the cap.

This is an incentive to move ahead with a stalled project to fit a low brake fluid warning light, which would then mean I would be more relaxed about the fluid level. So far I have acquired a float from a Rover 800 and have fitted it using Araldite to a spare cap. Now I need to wire it all up, and then I will write a thread about how well it works.

Roll on the Spring!

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