Last fall at the Texas FE. We installed a brand new in the box master on a maintenance truck Kwai owns. We also did all new wheel cylinders and such. We put the pressure bleeder on and were getting a little flow at the passenger rear wheel cylinder. Not much flow, so we upped the pressure. About this time is when we noticed DOT 5 was leaking out of the cab. There was a nice puddle of BFS inside the cab actually.

Turns out the new in the plastic wrapper master didn't have a piston installed. The dust cap could only hold the fluid back so long. None of us thought to look for a piston in the master before we installed it.

While I doubt that is your problem, it could be. More than likely your secondary seal on the piston has dirt or crud on it. Pull the master, pull the piston, clean everything, inspect everything and if all looks ok, put it back together. Starting with a new part, you probably just got a little bit of junk in there and no damage.

I would recommend BFS or DOT 5 which is the same thing. Less rust to everything.