B body question

So my dad recently purchased a 1970 roadrunner. With power brakes and front disks. The problem is the master cylinder keeps leeking in the front. It has wrecked all the paint on the inner fender. We tried putting new gasket material under the current rubber but it did not stop it. Any advice or links to new parts would be great. Oh we also had the brakes bled and cleaned up the edge of the cylinder