If the problem is the caster brakes locking up, let me explain a few things. The caster brakes are applied by a spring that pushes the plunger into the master cylinder that apples the brakes. When you lift your foot off the pedal, the spring pushes on the pushrod which supplies pressure to the caster brakes and applies them. You should take the floorplate off the truck, push the brake pedal down all the way, and make sure the pushrod that applies the caster brakes is completly retracted when you push the pedal down. I basically just shake the pushrod to see if it is loose. If if is loose, this means the rod is not in contact with the plunger in the master cylinder and not supplying fluid to the caster brakes to apply them. You may need to adjust the linkage to get this right.
If this linkage is not adjusted right, you will get the problem you are having.

The pushrod goes into a master cylinder through a washer that is held into the master cylinder by a snap ring. If you adjust the linkage to retract the pushrod too much, you will yank this washer and snap ring out of the master cylinder when you push down the brake pedal.

The master cylinders have a tendancy to have the plungers stick. When you step on the brake pedal, the piston in the master cylinder may have not come all the way out causing the caster brake to still be applied. This will also give a false indication of the linkage being adjusted correctly when the piston sticks. It could also be another cause of the problem you are having.

This truck has a funky brake sytem. The drive motor and caster brake are both applied by a spring when you lift the brake pedal. The drive motor brake is applied directly by a spring. The caster brake is applied hyraulically by the spring on the linkage to the caster brake master cylinder. The two master cylinders work opposite of each other. When one is building pressure, the other one is releasing it's pressure.