I am flushing the cooling system on my 1996 Suburban. The Chilton manual states my capacity is 20 quarts; the one source on the Chevy Forum states the capacity (without rear heater) is 17.5 quarts. Starting with a full cooling system, I can't seem to drain much more than about 3 to 3-1/2 gallons, and I'm mystified as to where the rest of the coolant is. I've drained the radiator, the heater core, and the block. Result was about 3.5 gallons, allowing for some spillage. Any suggestions/observations?

 

you will not get all the coolant out...the rear lines and heater core are impossible to drain. the reservoir volume is part of that 20 quart capacity.
hook a garden hose up to one line and run it until it runs clear and let it drain. add 1/2 of the system capacity with concentrated coolant...add water or a 50/50 mix if it stills need topping up...you should be guaranteed a 50%mix or slightly higher.

 

When the gravity dripping stopped, I connected a shop vacuum (running reverse pressure) at the thermostat port and blew out as much coolant as I could. I'm pretty sure I emptied the system.