Jim,
The motor and brake are electrically wired in parallel with each other and when you power the motor on the brake gets released to allow movement of the slide. The fact that you can power the motor and it turns without the brake being powered would indicate that the brake is not working otherwise the motor would be locked. You could wire them in parallel and try again. With no power applied the shaft should not turn and the brake should have that locked.
It is possible that the motor was shorted or the brake had things locked all the time and with power would not turn. That would give you the dim light due to a large current draw because the motor could not turn. That can also blow a fuse or kick a breaker. Best guess is that in the process of removing the motor and brake assembly whatever was causing the issue resolved itself so it now is working. I cannot say if it unit would stay that way over time or revert back to being problematic. What is clear is that the motor and brake assembly was the issue given no other scenario would dim the lights. Hope this helps.
Later Ed
