Master cylinder cap leak

[LETSRACE]

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.

 

[DartOS]

What do you mean by "run hard"? Cornering, fast take offs, fast stopping? The MC should not be leaking even under these conditions. Where is it leaking? If the MC has one 1/4" bolt in the center and that's where the leak is, I'd check the rubber around the bolt. The MC bolts were bad about attracting moisture and corroding. Doesn't take much rust around the bolt to let brake fluid escape.