leaking master cylinder cap.

First thing I would do is get a new gasket. Second I would be sure the level of fluid is low enough that when the car is on an incline, (perhaps the most incline you encounter) the fluid is below the lip of the cylinders.

Lastly clean and dry the lip of all fluid.
Clean the gasket of all fluid and debris.

Put the cap on as centered as possible and snap the retainer on. Let it sit that way over night.

Now drive around, hard stops, inclines etc. And check for leaks