Ford Repair: soft brake pedal, power brake booster, vaccum pump

Question
i have a 95 f 250 4x4 diesel 7.3 turbo it has a really soft brake pedal i checked the master cylinder seams to be fine i checked all break lines no leaks it isnt using fluid the vaccum pump on the engine is pulling good vaccum and when i disconnected the vaccum line going to the power brake booster the pedal hardened up like it should i havent had any brake componets loose so there shouldnt be any air in the system any ideas on whats wrong its got me stumped

Answer
pull wheels and check condition of brakes-worn shoes or wheel cylinder seeping can cause this
if that all looks good probably master cylinder-it doesn't have to leak to cause soft pedal just
bypass fluid internally. don't do anything before pulling rear wheels and checking brakes if shoes  are on rear.