I have the same 13" drums on my '75 K20 Camper Special, and the rears are grabby whenever the weather is cold and humid. However, it burns off after about two stops, and I then quit looking like an idiot who locks up the brakes at every stop sign. The other issue with these pads is that if I leave the parking brake set for a long period of time, sometimes it does not want to break loose when I first drive forward, so I have to rock back and forth a couple of times to get it to roll freely.
I replaced a leaking master cylinder and the front pads last year and it had no effect on the lockup problems.
I assume there is moisture that gets in the lining, and the material gets very grabby when damp. I had the rears done by a shop several years ago, and I don't know what brake shoes they used. Its such a PIA to get them apart with a 14 bolt FF that I don't really want to pull the drums.
My brakes work fine otherwise, so I've just decided that its infrequent enough that I can live with it. However, I'd be interested if you find a better solution.
Bruce