I take it you are talking about your Vstar 1300? I know you were working on a 650 for a while, which has drum brakes that could be loose for other reasons.
Have you tried sitting in neutral, putting the rear brake on and rocking the bike to see if any brake parts are moving?
I agree with Les, dont ride the bike till you figure this out.
And if you are testing while riding have to remind everyone: if you lock up the rear brake you have to keep it locked till the bike comes to a complete stop - if you let off while the rear tire has skidded out sideways even a little bit it will snap straight so hard it will throw you off the bike.
its the opposite of the front brake, if you lock the front brake release it instantly or the front will slide out and you go over the handlebars
if you lock the rear brake keep it locked and steer into the skid (hopefully you will not skid into anything) until you stop, or it will throw you off the bike.
one other thought: if you lock up the rear brake with the clutch engaged (not pulled it) it will stall the engine and make horrible sounds