ay mate,, how long since you pulled the rear assy apart,,
if not for a while, might pay to go thru it and have a look,, could be the small tin plates that seperate the steel of the brake pad and the cast alloy,, may have lost their spring,,
or you may have a small stone stuck in the groove of the fibre, going backwards puts the pressure on a different spot of the stone slightly moving it out it,s forward rolling position