My 87 accord 2.0 exi with alb failed its test re rear brake balance.

Both service and parking brake figs are way off balance do to some problem
on the right rear disk.

Thing is although considering myself reasonably confident I cannot find the
cause.

Cable is operating correctly
Piston is perfectly free and self adjusting handbrake mechanism work
perfectly and yet performance is way down.

Before I strip down the calliper for detailed examination is there anyone
out there that's come across this???