First check if there is oil, grease, brake fluid etc. on the shoes and/or drums like an earlier reply suggested.
If the shoes and drums are OK, check the size and position of the brake shoes at each wheel because sometimes there are longer and shorter shoes and if I remember correctly the shorter shoes go toward the front of the truck.
These must be in the correct position relative to wheel rotation as they are 'servo' type brakes that help to apply the brakes as the drum rotates which helps to reduces pedal pressure.
If the shoes are not in the correct position, the brakes will 'self apply' when you 'touch' the pedal and act like you describe.
