No need to get that involved. If you drain everything, all you'll be flushing out is air and that isn't very productive. Leave the radiator cap off, then run water through in every direction from a garden hose. You only have to remove one heater hose, preferably from the engine so you don't risk breaking the nipples on the heater core, especially if they're plastic, a GM favorite. Force the water into the end of the heater hose, then into the port on the block or head it came from, then into the radiator or reservoir. Once no more red or green is coming out and there's little signs of dirt, drain as much of the water as possible. You might have to get a flow going from the petcock, then use the vacuum to draw water out of the reservoir.
The whole cooling system only holds about two gallons and as you found out, there is going to be some water left in places that you won't get out. Fill the system with one gallon of new antifreeze but don't fill the reservoir. Warm the engine up and watch the temperature closely. If the upper radiator hose doesn't get hot, the thermostat is staying closed because air has pooled under it. They need to be hit with hot liquid to open so you will have to bleed that air out. Once bled or circulating, air pockets will work their way out and you'll have to add more liquid. Start with straight water, up to about half a gallon, ... Just enough to maintain circulation.
Once the coolant is circulating normally, check the freeze point, then add water or antifreeze as necessary to the reservoir. That's why you don't fill the reservoir right away. Most people will discourage you from using the red Dex-Cool antifreeze. We call it "Dex-Mud". It's good maintenance to flush the cooling system every few years. The antifreeze doesn't change. It's the additives that wear out and acids build up in it. The acids are what cause radiators and heater cores to corrode.
We never remove block drain plugs. There just isn't enough time left in our lives for things like that. If you did, the water from flushing would run right out and never get up high enough to reach many of the places casting sand can be stuck in the block.
Wednesday, October 24th, 2012 AT 7:50 AM