I'm trying to drain the coolant from my Kohler 15RYG. I opened the radiator cap, opened the drain valve and collected about 2 gallons of liquid. The book says the capacity is 3 gallons. I filled with clear water to flush - it only took two gallons. I drained again, this time opening the cap, the drain, and removing the lower hose. Still got only two gallons. It's clear that the first draining wasn't complete because the second draining had a fair amount of color from the anti-freeze. How do I completely drain the coolant?