Sorry for the late reply. One of my three Pro-S backs did the same thing recently, and for the same reasons as you I decided it was not worth it to send it in for repair. So, out with the camera repair tool box and let's see if I can fix this thing. It CAN'T be all THAT complicated, just a film counter and advance ratchet.
Not so fast, my friend. My eyes got big when I took the cover off. That thing looked like a Swiss watch inside. Fortunately, mine had an obvious problem: the main cast metal piece the advance lever and return spring drop into had one of it's two large "arms" broken off. Easy fix with JB weld. Reassembled and tested, but although the film counter and advance now work (for three or four frames, at least) the back jammed after that. I haven't had the moxie to take it apart again, but trust me, that is NOT a simple mechanical assembly in there. I know my limits and I'm not Jimmy Koh, so guess I'll fumble along with my remaining two backs but start looking for a replacement! I also have wished for a repair manual for the backs, but not sure that animal exists.