Master cylinder cap leak

I have manual drum front and rear brakes in my 75 Duster, the cap for the master cylinder leaks after I run the car hard. I replaced the cap and gasket, that didnt work so I got a rebuilt master cylinder and put that on that didnt work. I noted a small space (groove) for a vent in the cap so the rubber could move up and down with the fluid level. I filled that with black RTV and drilled a small hole in the dome of the cap. Im looking for a cap that is flat to seal the fluid. Or is a race master cylinder needed to keep fluid from leaking out.