Rear Axle brake imbalance -

You say "wheel cylinder" - was that just one, or both sides? If it was just one, chances are the other, old one isn't working as well as the new one, which is causing the imbalance.

Hopefully they did the pads on both sides, as they must always be replaced as a full axle set.

The likely causes of differential braking are:

• hydraulic imbalance (air in system, failed proportioning valve, etc)
• ABS unit not properly bled
• different co-efficient of friction on brakes (one pad glazed, the other not, as an example. Could also be a difference in finish on the rotor/drum/metal brake surface.)
• dirt/debris/stones interfering with braking action

For resolution, I would probably first visually inspect, then bleed the hydraulics. If it was still not balanced, then I would pull the pads and sandpaper them to a fresh surface. For starters.

For testing, I would jack up the rear of the car, and with a breaker bar or similar leverage try turning the wheel and see if the drag with uniform brake pressure (could be engine off) is about the same.