My brother has a 1996 F250 4x4 that he pull a fifth wheel with at times, after his recent trip he checked his rear brakes for peace of mind. He found one shoe broken in half (WOW), so he replaced the rear shoes both sides and bled the brakes. Now his brakes work good forward but when he puts it in reverse the brake light comes on and peddle goes almost to the floor. He say it will stop but you have to get on it to get it to. Any ideals of what it could be. Any help is always greatly appreciated.