Adjusting Drum Brakes
Jack the rear end of the car up (use jack stands if you have them)
Locate the notches (2 on each side) on the inner brake backing plate (look at where the brake lines attach) Use a brake adjustment tool (or a flat blade screwdriver) and adjust by inserting screwdriver into the slot and prying in one direction. Repeat until you can tell the shoe are adjusting out word toward the drums There should be resistance felt.
Adjust the shoes until they touch the drums and you can barely
turn the drum
by hand. DO both sides.
Get in the cab and pump the brakes - this will center the shoes.
Try to adjust the shoes again. once they are tight AFTER pumping the brakes,
back them off a few clicks. (usually 2-4)
If the pedal felt good during this process, you have found your problem, otherwise check for leaking brake fluid. If there is leaking brake fluid then you need a new Master cylinder. Also check the Brake cylinders on the drum brake housing, they tend to freeze up if they are old and corroded.(then replace)