there is an attactment that fits in between the lines. as you can see there is like a cross:
to compress this you need
the one with the two indents as shown in the picture are the ones you need to fit in the cross to then compress,
and instead of going straight back like the front wheel, the rear caliper twists like a screw and then compresses.
hope this helps
if you do not have one of these what you do is stick a screwdriver that will fit in the indent and twist it clockwise.
