leaking master cylinder cap.

Whats up guys? I have a 67 dart that I put 72 disc brakes on, every master cylinder i've had on it leaks around the cap. I tried a bunch of stuff to make them stop leaking but no luck. it drips down and leaks in my TTI headers. any advice? i was thinking maybe a plastic reservoir with the screw on caps but not sure what would fit. I run the brakes manual