Rear brakes imbalanced across an axle

Helo,
My Jimny has failed its MOT last week for “brakes imbalanced across an axle Rear (Axle 2)[1.2.1(b)(i)]”
As advised by the mechanic, I have adjusted the shoes on both sides and cleaned everything up while I was there. When I took it back the test showed 60% on the nearside and around 5% on the offside although the handbrake works fine. 
Anyone have any ideas what it might be?
Thanks in advance

It sounds like the slave cylinder is either stuck or dead. If there is no sign of a leak then you might be able to free it off, just don't let the pistons come all the way out as it will make a mess and you will have to bleed the system. Alternatively for the price just get a new one and use it as an excuse to replace the fluid, though be aware that it's likely the fixed pipe is not going to appreciate being disturbed so have a length to hand to make a repair section. In other words expect mission creep.