91 ford f350 dualy brake question

I have a 91 F350 dualy 7.3 d...... When I press and hold the brake, it goes slowly down about half way, then it will hold solid at that point, my car did that one time and replace the master cyclnder and it fix problem, but I replaced the master cyclnder on the truck and it is still the same, feels like it has a bleed back when holding presure on it, and it slowly goes down about half way and then stop...no lost in fluid.... not sure what else would allow this type of bleed back.