Check for cracks, check for dirt/debris on top of the reservoir that could cause a gap. Are the brakes still working okay? If there's some sort of blockage in the line it could be putting too much pressure into the reservoir fluid and blowing out the gasket anyways. Also maybe check to make sure the air breather into the space ABOVE the gasket isn't plugged - if so, pressure could be building up when you RELEASE the brake pedal.
I actually just bought a new master cylinder, but mine was leaking between the cylinder and the booster. Hope you figure it out.
Jeff