As elekie doc notes, if the master is leaking then it's possible air is entering the system, as he correctly says bleeding the brakes might correct the imbalance.
The problem with just looking inside the drums is you don't know if everything is moving correctly, if you have a partially seized slave cyl you won't know till you try it, also is the automatic adjusting mechanism working correctly both sides, or has someone adjusted the handbrake cable up too much wihc can sometimes stop the auto adjusters working correctly....if JohnF reads this thread he's the man who knows about keeping older Focii going for next to nowt.
How you check that is with the help of a competent assistant, with drum removed you hold one brake shoe in place with a screwdriver wedged against the edge of the backplate, and get your assistant to press the brake very gently till you see the other shoe moving, then repeat with the other shoe, that proves the slaves are free and working, only remove one brake drum at a time.
You can check caliper pistons similarly, by getting an assistant to pump them out whilst you watch at the sharp end.
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