leaking master cylinder cap.

Lots of good ideas already mentioned like flattening the mating surface of the MC with a file.

As a last resort, maybe you could get a second gasket and cut it apart leaving a half inch or so around the circumference. Use that to double up the clamping area.

You might try contacting the Ramman. Maybe he could shed some light on this.

I run DOT5 in all my old cars. One of the advantages is that leaks or spills don't eat the paint, just wipe it off.