I am trying to drain and replace the antifreeze in my truck.

I have located the drain cock, but when I turn it 90 degrees counter clockwise it stops and no antifreeze comes out.

What am I doing wrong? Also what is the best method to drain the complete system?

Obviously, you need to remove the radiator cap first. If the opened drain is plugged, take a small rod and poke it. I have removed block drain plugs on vehicles where you need to do this; then, the coolant gushes out.

I would drain the radiator and then remove the two block drain plugs. If one of the drain plugs is near the starter, disconnect both batteries first. Refill with a 50/50 mixture of approved antifreeze (likely G-05) and distilled water. Finally, add 8 to 10 oz. of Supplemental Cooling Additive (SCA), to help prevent cavitation. Most SCAs do this by transforming the soft ferrous oxides (rust) on the liner into Fe3O5, a very hard substance. Every 15,000 miles add 8 to 10 oz. of SCA and test with Fleetgard test strips. If you don't do this, cavitation will cause pin-size holes to develop in the cylinder walls and your engine will be ruined. Normally, these holes first develop in the two rear cylinders.