My brakes will work fine on short distances but.

My brakes will work fine on short distances but when I drive over a mile they get mushy and the pedal goes to the floor. I have replaced a leaking hose and then replace the front brake pads, did a bleed and still mushy. I replaced the master cylinder and bleed the whole system. They work for a short distance ( firm and strong) but when I drive over a mile the pedal goes to the floor. If I pump the brakes they work again but if I drive a bit they go mushy and to the floor again. Any help would be greatly appreciated!
Semper Fi
Phil.