How to fix rear brakes on a Saxo VTR?

My car recently failed an MOT on offside rear brake producing little or no effort. I had to replace the caliper, discs and pads on the rear of the car anyway, however this has not resolved the issue. The brake system has been bled a few times and this has had no effect on the brake. Also when the offside rear is off the ground the brake seems to catch however when on the roller tester it produces no effort. I bumped the car into the curb earlier in the year due to the snow and both the offside brakes are weaker than the nearside. Any ideas?