First thing would be to remove the wheels and check the rear brake pads.
You also need to check that the calipers are sliding freely on their slides and that the brake pads are free to move and not jammed.
Next check that the handbrake cables are free to move and not knackered.
Next check the operation of the handbrake levers on the calipers, when the handbrake is applied and then let off they should come fully back to the stop post when the handbrake is in the off position.
If the leavers don't come fully back to the stop post you either have the handbrake cables adjusted too much or the levers are starting to seize.
Try pulling them back to the stop post, if they go but are tight they are seizing up.
If the levers are seizing up you can sometimes buy yourself some time by working the levers back and forth whilst squirting release
into them, but im my experience they soon seize back up again.
Check it out and see what you find.