disc brake drag hayes circa 2003

Have older trek fuel circa 2003 w/hayes disc, which I believe to be
hfx mag or hfx 9 but wouldn't bet my life on it. These brakes (front
and rear) have always had a little drag on them, which I don't believe
is normal and which I would like to fix instead of changing them out
to v-brakes or something else. I believe the pistons are all the way
into their cylinders; I can't push them back any further. I don't see
any wobble in the rotors. It's as though there's not enough clearance
between the pads and the rotors. Looking for suggestions. Thanks.