I am having an odd problem with my brake master cylinder. I am loosing fluid, and it appears to be leaking out the top of the reservoir where the lid sits. I can tell, because when I run the car, the outside cylinder gets wet with fluid, starting from the top down. What could be causing this? I am hesitant to replace the MC because I don't know if maybe the vacuum booster is leaking and causing the fluid to leak out of the reservoir? The brakes work just fine. The MC had been replaced about 5 years ago, but the car hasn't been driving much since.

Cast iron M/C? or the newer style with the plastic reservoir?

Plastic? Some have had problems with the plastic ones leaking because, IIRC, the caps wouldn't seal on M/C. Top plastic still had some seam flashing that left uneven surface that cap gasket couldn't handle.

Cast iron? Do a search here. Steel spring that holds cap on (or is yours bolted down?) may be weak or gasket shot. Gasket doesn't have any holes in it, does it? It should have a diaphragm that allows fluid level to go down some with brake pad wear.

It is cast iron. I was thinking that the lid may not be on tight enough, or corrosion on the top is preventing a good seal. But why would brake fluid be getting up there in the first place? Does a small stream shoot upwards when the brakes are pressed?

When you operate the brakes you do get a squirt of fluid.

Some master cylinders will shoot some fluid up when applying the brakes, all of them will release the pressure and fluids compressed back into the master cylinder when you release the brakes

Not from the booster & there is normal slosh. check that the thin rim around the M/C perimeter is clean with no minute debris that can effect a leak, that the gasket where it contacts the MC/lid is clean/smooth on both sides, same reason & might need to tighten the bail wire

Do u have a chrome lid on it? I had a problem with one, and the gasket would seal to the master cylinder, but not to the crome lid, and fluid leaked past it.

i have 2 that are leaking as well but havent taken the time to figure out why.they are both new,cast iron bendix parts store cylinders with stock style lids.i suspect the bail clips are not as tight as they shoud be or the rubber gasket is too hard ?

I pull up on the spring holder makes it a bear to get on and off. I ran into one that was emptying the rear cylinder and I could find no reason for it so I went to adapter and aluminum master cylinder works fine {New Mopar}

I have yet to replace a MC with a stock rebuilt cast iron one that didn't to leak. I always check the seal and top edge of the resevoir but eventually they leak. It is truly agrivating and it is time to change resevoir types.

I'm wondering if lowering the fluid level slightly would stop that. Within reason of course. If you have plenty of reserve (fluid height) & you check it regularly I think it'd be safe & worth a try