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right side rear brakes are 30% down compared to other side. Fitted new pads both sides, bled cylinders, brakes not binding. Mitsi Grandis Royal 1999.

So, presumably, you're asking what the problem might be?
There are numerous possible causes, including a lazy caliper, partially collapsed brake hose, partially seized calliper slides etc, etc. However, I'd suggest the most likely cause is the brake proportioning valve.