Thanks for the explanation, that is helpful.
My situation specifically goes like this.
Bought the truck in Apr. '07. When I bought it the guy said that he had flushed the radiator numerous times and could not get the "rusty" stuff out. I replaced the Termostat a few weeks ago and it was covered with rusty dusty stuff. The hose from the reservoir tank was completely plugged with the same stuff. We cleaned that out and figured this spring we'd need to replace the heater core and radiator. I never thought about the water pump. My concern is that when we opened the pitcock, literally a trickle came out it was so plugged with that stuff. I'm assuming when we open the drain plugs on the block we'll run into the same problem. When something is that built up, can I ever hope to get it all cleaned out? I'm not getting much heat and figure it is probably because the heater core is plugged to some extent based on how plugged that hose was to the reservoir. When we replaced the thermostat we tried to flush the system the best we could, but since the pitcock was plugged we just unhooked hoses and tried to get as much out as possible. When we filled it, we put three gallons of anti-freeze in and it was full. I'm worried because it should hold 5.5 or so I believe, I realize we didn't get it all out based on how we went about it. I just picture a block plugged solid with crud...
Any thoughts would be appreciated...