I just put a new master cylinder in my truck about two months ago because I was leaking fron the front port (rear brakes) and i could see it was dripping from the lid. I put a new master cylinder on, filled it with new unopened dot 3, and bled the brakes out. worked great til the other day I noticed I was leaking again out of the lid because it was rusting the new master and I could see where it was coming out from the front part once again the rear brakes, its almost empty now so ill have to bleed them again once i figure out whats up, any input as to why this keeps happening would be very helpful, thanks.
I recently did a M/C on my 86 and went to 2 different parts stores to find the rebuilt M/c's were both filthy and rusty out of the box from the rebuilder, even the rubber gasket was caked with rust flecks. I bought one anyway...not sure why as 10 years ago I would have laughed and told them where they could put it. They used to treat them with a clear lacquer to prevent corrosion. I guess it is the nature of quality we are faced with in rebuilds now, and the cheapest parts stores seem to win the game, bean counters get to spec our parts now. I am not speaking of all rebuilders as there is still quality out there but getting scarcer.
For your M/C I would start by taking the lid off and removing the rubber gasket and cleaning it, get it as clean as you can. Wipe the moisture off the inside of the lid while your in there. I would take off both lines and inspect the press in orifices at the cylinder ports they put in that act as the seat for the flared line, make sure they are in straight and look like they have seated well. Slide back the threaded nuts if you can and really give the flares a good look. They have been known to crack where the nut barrel makes contact. I would look closely at the area under the M/C at the booster for any residual fluid. Use a flare wrench so you can get the fittings tight. I would refill it and clean it all with a little carb or brake cleaner after it's filled with the lid on. Pump the brakes good and then check for leaks, it has to be dry and clean so you can isolate the problem and find the fluid leak point, I find a dry finger never lies. As far as the cylinder rusting, the fluid should not cause the rust, something else must be going on. I have never seen fluid pushed out of the M/C lid, if you have blockage in one of the lines, you would know that during bleeding. You don't want even a dab of fluid showing up, anywhere it comes out air will go in.
