1981 GPz 550 Rear Master Cylinder and Front Caliper.

Congrats, sounds like you got a pretty good deal, 2 brake caliper pistons alone would be around 70.00...so you did good
Pete

Any tips on how to get the rear caliper bled? Seems like I have air stuck somewhere.

Is the small fluid return port in the master open? Assuming yes, it helps to pull fluid through the system by providing a vacuum on the bleeder nipple. If you don't have a vacuum pump than a long piece of clear tubing that you can suck on with your mouth is a reasonable alternative. Just watch the tube and stop sucking when fluid enters the tube. This pulls fluid though the system and primes the master. Typically, once you get the system primed then you can bleed the system conventionally after that.