Remove the radiator cap and make sure the radiator itself is full. Do it with the engine cool, and start the truck to bleed out any air pockets as you are filling it. Then check the over flow tank (follow the small hose under the radiator cap back to it). It has a cool level mark and a hot level mark, fill it based obviously on what temperature the engine it at when you fill it. If the truck is full of 50/50 anti-freeze/water mix, you may have a bad thermostat, water pump, or radiator cap.