Draining/Flushing Coolant

If I use the prestone flush kit, is it really neccesary to drain the block? Or will the flushing chemical break everything up and drain out? Do i keep filling it with water and leave the drain open? Thanks

What I usually do is get the backflush kit and the 7 hour flush, drain the system and refill with water, add the flush. Do the 7 hours. Cut the heater hose and put the T in, hook up the hose to the T fitting. Warm it up again a bit, not too hot, cap off and the redirect thing in the radiator filler hole, I put another hose on that to get it away from the jeeps nose. One other thing I get is one of those little screw on fittings with a ball valve in it so I can turn the hose on and off right there, they are about 2 or 3 inches long with a little lever on them. Once the engine warms up a bit I turn the hose on slowly and start back flushing it, turning the water off when the tstat slams shut.
The only other way I can think of is to disconnect the top hose to the radiator, make a fitting up that will reduce that hose size to a garden hose 3/4" size then just keep adding water to the radiator for a half hour or so with the heater on full hot.
Once I have it good and flushed I drop the lower hose, let the by then pure water drain out, reconnect the lower hose, add a gallon of pure antifreeze as there is about a gallon left over in the block and start bleeding an filling.
If you are going to do it then do it before it gets cold again.