75e15 rear brakes come on on their own !?

i can do a mile or so after leaving home and rear brakes gradually start to come on without me having even used them.half operating h/brake several times while moving usually frees them,if not they become so tight that i have to pull over!.
calipers have been stripped and freed and sliders copper greased.
could it be air pressure or master cylinder problems?
Cheers