[redeye280z]

I am in the process of changing my coolant expansion tank and decided I would flush the whole system while I'm at it. I followed the instruction in Pedro's Garage but I only managed to get about 12 litres out of the car. The car is a 2000 Boxster S and from what I understand there should be about 18 litres in the system. what am I missing?
P.S. I disconnected the 2 big hoses as part of the draining and the drain screw.