Do the brake linings appear to be nice and thick? Maybe thicker than the age and hours of the truck would suggest?
If so, it is possible that someone put brakes on it and the truck simply has not had enough real use to wear the new shoes into conforming to the drum.
If the shoes are not all that thick, then the idea that one of the pistons in the wheel cylinder might be rusted stuck to the cylinder bore moves higher on the list.
But, it is unusual for this to occur except in trucks that are left out in the weather all the time, and are left sitting unused for long periods.
More questions. Does this truck use the hydraulic system to operate the brakes? Or do the brakes have a separate master cylinder and brake fluid?
If the truck uses the hydraulic system for brake power, its possible that at idle RPM (on a truck this old) the oil pressure of the hydraulic pump may be just a bit down from ideal and that may be requiring a heavier foot to actuate the brakes.
I have seen trucks with the hydraulic system powered brakes that seem to have weak brakes, until you rev the engine a bit, which increases the oil output of the pump. But increasing the oil output by raising the RPM also makes the truck harder to stop unless you are (properly) using the inching pedal (left pedal) to stop.
If it has hydraulic system brakes and they seem weak/meek, I would also suspect a defective brake pressure accumulator.
On trucks having this type of powered brakes, the oil pressure accumulator is supposed to do 2 things. It is intended to be part of the redundancy requirement to have a "plan B" brake system if the engine or hydraulic system fails while the truck is operating so you can stop the truck. And it is also intended to help make up the deficit in oil pressure if you step on the brakes when hydraulic oil volume is low, as when other hydraulic functions are being used at the same time as brakes are applied.