Diggers,
Well, I harken back to my GS 550E of the same era and it had a brilliant rear disc. When drilled and fitted with soft pads it was an amazing system, but I'm off track here. The only adjustment I'm aware of is the "butterfly" nut on the end of the brake rod. Haul your R65 up on the centre stand and turn that nut clockwise while at the same time turning the rear wheel in the direction it would spin while riding 'till you feel the rear wheel has some resistance to being turned. Not a lot, just a bit of rubbing or drag should be felt. Your rear brake pedal should have some play and when you are astride the bike you should feel the pedal is quite firm without pressing too far. thats the adjustment bit. There is a lot more that can be done to improve the rear brakes. I'll let others chime in on Rear wheel removal, pad inspection, cleaning, oil / grease contamination and so on. I was advised that by simply using the rears more they would improve and that worked for me but when I first asked a fellow at a tech day about the faint power or the rear drum he replied "Oh thats normal".
rich
Who always thought a disc on the left side would be the cats a$$