Master Cylinder Burbing

[hexafamily]

1972 C10 with factory power front disk brakes. All brake components have been replaced except booster and proportioning valve. My problem is fluid keep leaking out from the top front of the master cylinder cover. The previous master cylinder did the same thing, which is the reason is was replaced. I have noticed the front brakes are weak, and the rear tends to lock up easily, and under hard braking you can not get the front to lock up. Sometimes the brakes feel like coaster brakes when come to a stop at speed. Pedal is firm, but not hard.

Any ideas on whats happening?