i need ideas as to why my brakes keep going shit.im getting air in the system,when i bleed them they are fine for a couple of days then start going to the floor.iv changed the master cylinder,and brake cylinder and still no joy.it is not leaking anywhere as far as i can see,plus im not loosing any fluid,if it is, it must be very little as i havnt noticed! its an escort van 55d. please help as its bugging me.