Leonard,
Try this, I take it the cap has been for a bit, pop it off and check the gasket surface for a tiny skip on the sealing surface maybe there is a flaw on the master cyl. sealing surface? the gasket should show it.
A properly sealed and vented cap should not leak even with a few bubbles in the system, that is what the bladder and vent are for.
just my 2 pennies.
Tim
