Hello Neil, I am not familiar with the 500 models, but.......
If they are anything like the CB750's, you must undo the brake linkage to the rear wheel. On the 750, there is a round rod that is spring loaded with a nut on the end to keep it in place. This must be undone....... remove the long rod from its retainer, so that it does not bind, put the spring and nut in a safe place to refit them after your chain adjustments.....
Once again, also on the 750, there is a flat brace that connects to a bolt through the brake drum itself, to keep the drum stationary when the internal pads are pressing against the inside of the drum. This brace/bolt must also be undone......... remove the brace from the bolt, do not undo the other end of the brace; it's best to leave the bolt in place on the drum.......
These two components must be temporarily removed so that the rear wheel is
free to move forward or backward, once the axle is loosened, and the wheel may move without binding.....
This is true on the 750.............. I do not know for sure on your bikes........ but, with this in mind, you can take a look and see if your bikes have these components.......
Good luck..........
~ ~ ~ jaknight ~ ~ ~