I changed the coolant on my B 2620 as the manual recommends at the 2 year mark. Manual says capacity is 4.1 gal, I take radiator cap off, loosen petcock, drain, only about a gallon and a half comes out, flush with running hose for about 10 minutes. Refill takes less than 2 gallon of coolant mix. What am I doing wrong? radiator was full when i started and full when I finished, got over 2 gallons left?? No problems but don't seem right.