To properly flush the radiator coolant requires cycling the new fluid in as a process. Simply draining the radiator does not allow all the coolant to drain out of the engine, heater core, etc. The only way to do that correctly is to:
1. Drain the radiator and refill it with distilled water
2. Run the truck at normal operating temperature until the coolant from the engine cycles into the radiator; once done, drain the radiator again and refill with distilled water again.
3. Repeat the normal operating temp running until the coolant in the engine is cycled to the radiator again, drain and refill the radiator with distilled water again.
4. Run the engine again at normal operating temp until the coolant/water cycles to the radiator again. Drain that and refill the radiator with the new coolant.
5. Run the engine again at normal operating temp until the coolant/water cycles to the radiator again and drain out the radiator and refill again with coolant.
You should repeat that last step until you have used all the fluid. Flushing the coolant isn't a simple matter of drain and refill, it is a cycle that must be followed to remove all the old coolant from the engine.