Rear brake problem

Im brand new to motorcycles and my 1977 kz 650 is my first bike. my rear brake will engage, but will not disengage. any suggestions?

the piston in the caliper is dirty and will need to be removed and cleaned and also the caliper will need to be cleaned and all the old crystalized brake fluid will have to be cleaned out of the seal grooves the a new seal kit will have to be installed and while your at it replace the brake pads as they have probably worn out enough that the pads arent returning smoothly. make sure you get a factory manual it will talk you thru the steps involved.

Would also refurbish the master cylinder, because the pressure relief orifice might be clogged. That's the mechanism whereby line pressure is supposed to be relieved when the rider is no longer applying the brake.

Good Luck!

Thanks guys, I will do all you said and hope for the best