You sure it's not an axle seal leaking? Gear oil. That often leaks onto brakes at the wheel.
Are you asking why or where its coming from or telling us. If its drum brake rear its probly the cylinder leaking. Also could be a bad hose. If its disc rear again it could be a leaking cylinder or bad hose or connection. Pull off the wheel and have a look. If its drum then pull the drum off also. You will see if its a leaky cylinder. Check under your car too at the rear wheel might be something that side.
Probably the wheel cylinder. Remove the brake drum and take a look. Easy to replace but you will need help bleeding it after the repair. Won't need to bleed the others if you don't let the master cylinder drain completely during the repair.
Fixed brake pipe rusted through, flexible pipe perished and leaking, or brake cylinder seal gone.
Apparently.
