I have a 1994 f150 4x4 that has been sitting for 2 years. Started driving it again and have brake problems. Replaced all brakes and bled all lines but do not have a good firm pedal. It goes to the floor, adjusted brakes again and bled again, same thing. Could it be in the master cylinder?

 

Have you thoroughly checked the brake lines for leaks??? if so, it sounds like maybe the seals in the MC are bad. I'd replace it.