Had a Kia cerato in today, neither rear brakes locked & both made around 200 kgs but I had an imbalance figure of 43%. The fronts both locked with good figures but had an imbalance of 35%. VOSA gave it as a pass so I went in to "brakes"-"brake performance" then "imbalance" & failed it that way. I did try earlier in the day to run both rollers as Tony said but once I get a lock my brake tester just gives me an imbalance figure with no efficiency readings.