As part of my finally getting around to servicing the bike I bought at the end of last summer I went through and bled the front and rear brakes as well as new pads in the front to solve an extremely weak front brake setup on my 89 100RS.  While the front now feels closer to 85% of where I expect it should be, I noticed today while changing the final drive oil that the rear disc is dragging to the point of being difficult to turn by hand and impossible in reverse and unevenly throughout its travel at that.
Setting aside this likely means I'll need to have the rear reground or replaced at some point, could this dragging be caused by the tension spring (35 21 1 457 049) for the rear master being missing or am I likely going to have to disassemble the rear master to fix this?

It's probably a clogged rear master cylinder, that does not allow brake fluid to flush back.

Hoses containing extremely dirty fluid, deteriorating inside passages, or plugged fittings also could be blocking fluid movement. If your hoses are the rubber originals, they are the usual suspects but faulty caliper piston seals can seize brakes, too. Disconnect the brake hose from the caliper, squeeze the hand lever and see if anything oozes from the hose.

Old hoses can hold a multitude of problems.  Would be good to replace them or at least disconnect them at the calipers and do a good flush with new brake fluid.

I'd lose the rubber hoses and go with steel braided lines first off. This change made a big difference in the stopping ability of my K by eliminating the ballooning of the rubber hoses under pressure and also gave me peace of mind. Those rubber hoses break. Had that happen in a car more than once. Brake fluid is dirt cheap and easy to flush. I do mine once a year