leaking master cyl

[airbrusher]

i have a 72 c10 shortbed that was a 6 cyl three speed all manual that i have converted over to a 350-350 combo with ps and pbs my prob is my master cylinder leaks out of the cap i am on my third one and no change if i drive the truck any distance 10-20 miles there is some fluid that has gotten by the rubber gasket on the lid not alot but any is to much does anyone have any ideas? the brakes all work great and like i said i am on my third master so im pretty sure thats not the prob thanks!!!