Truck Forums - Soft brake pedal

Hi Everyone:
My name is Tony and i'm new to the forum,looking for help on my 92 F350,4X4,Automatic Diesel Dually.My problem is,when I run the truck it seems that the brake pedal is weeping down, it weeps down to a certain point (allmost to the floor) then stops,but i've got great brakes.I have replaced the master cylinder,two new rear seals,very gently pinched off each brake line,I even installed plugs in the new master when I installed it,I also replaced the RAV valve,and I still have this problem.I contacted my local Ford dealer and talked to the service manager and he told me that this has been a problem since the late 80's?Apparently it is more a problem with trucks running the diesels than the gas engines.Some one please Help haha!