Master Cylinder Mystery leak

I have a'65 Valiant with a 318, it has a aftermarket dual master cylinder, I've noticed that when I shut off the car after a cruise, the master cylinder will leak a little fluid..I noticed this because the fluid is dripping onto the exhaust manifold and burning up..The two brake lines are perfect without leaks. Somehow its coming out from beneath the rubber gasket. Could his be from the heat? Any thoughts are appreciated.