If you have brake fluid leaking on the left rear wheel, the most likely cause is a wheel cylinder. It is also possible that a brake line is leaking at the wheel cylinder, but the wheel cylinder is probably the problem. Usually, wheel cylinders will wear out and begin leaking. This is relatively an easy repair.
There are on two bolts that hold it on and a brake line connected to the wheel cylinder. Once you pull the tire and drum off you will see the rear shoes and the wheel cylinder is located at the top between the two shoes. However, sometimes you have to remove the shoes to easily access the wheel cylinder. Once you access it from the wheel side, remove the brake line and the two bolts. Remember, when going back together to make sure the brake line and bolts are tight and BLEED THE BRAKES BEFORE OPERATION. I hope this helps