03 transit 350 brakes sticking on?

i have 03 350 lwb transit and my front brakes seem to come on by their self once you travel for about 1 hour the pedal get hard as if build up of pressure and slows it right down.then can be ok for while then it happens again i have replaced the flexy hoses,(as was told that they may have clappesed inside) discs, pads, bleed it twice garage can,t find soloution HELP please