I changed Master cylinders a while back when I had brake issues. They are/were both later model plastic reservoir bodies with the black plastic caps. Now for some reason I can't get the caps to stop leaking. I've reduced the amount of brake fluid in there. I've swapped out the rubber cap inserts, tried front to back and back to front. they still leak. Anyone else have this problem and a solution???
DC.

There is like a channel in the caps for venting purposes I think. I would always make sure that after the caps are screwed on the this is toards the front of the car. It's a raised area on the cap.

Ok, so the vent goes in the front? I currently have it on the back and facing back. Only one cap with the vent in it.
DC

Had the same problem with mine, so I just used an old antifreeze plastic jug, cut some circles out of it to just fit in the caps, under the rubber grommets, and have not had a leak since, try this I bet it will stop the leakage problem.

Dave

good idea

The vent in the cap allows atmospheric pressure to the top side of the rubber grommet. As the brakes wear and the fluid level drops the grommet also drops and displaces the area previously occupied by fluid. At no time is the fluid exposed to the atmosphere, for brake fluid is hydroscopic. It absorbs moisture from the atmosphere like a sponge.

So...if it's leaking it's not because of overfilling. It's probably because the fluid is contaminated with moisture resulting in fluid expansion when hot,pushing past the rubber grommet.

Flush all the fluid out and refill with new fluid from a sealed container.

BTW. DOT 5 fluid tends to form microscpoic bubbles when pumped quickly, these can be difficult to eliminate through bleeding. Use DOT 3.

I can tell ya it's not over filled because I removed a lot just to see if it would stop leaking. As far a contaminated, it doubt it because the whole system is new and has done it from the beginning.

Mine on my demon does the same thing, Ill try making the vent to the front also, THanks guys...

Kasey

I made spacers for under the rubber seals from the lid of a cool whip tub.Worked great,no more leak after every drive. Chris

Having the same problem with mine.....I will "see what I can see" with the vents and home made spacers tonight.

Good info here! I've been having this same broblem, too. Real annoying!