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Does the e brake pedal go to the floor, or does it not move at all? Maybe it will ratchet a few clicks, but not hold on a hill?
Check the condition of your e brake cables. They could be seized (from non-use), broken, or stretched. There is one cable from the pedal, then splits to two at the back of the truck. Slack can be taken up with an aftermarket device, found in the 'Help' section of the auto parts chains.
Broken cables have to be replaced.
If there is good tension on the pedal but truck doesn't hold, there might be an oil contamination on the rear brake pads, from either leaking axle seals or leaking brake cylinders. You must clean shoes and drums, and maybe even have the drums turned and shoes replaced so there is full contact for maximum friction.
I have even seen trucks that were missing brake hardware, specifically the e- brake equalizer bar.
Look at all these possibilities and try to narrow it down.
Mark