Why is brake fluid leaking from driver side rear wheel on a 91 Buick Century?

There's only two things it could be: the brake cylinder or the hose. It's probably the brake cylinder, and those are easy to replace: jack up the car and put it on a jackstand, remove the wheel, then the brake drum followed by the shoes. Remove and replace the brake cylinder, reassemble the brake, bleed the system (you should only have to bleed the passenger front and driver rear since they're completely separate from the driver front and passenger rear due to the "dual diagonal" brake system cars have) and put the wheel back on.