I have done a refurbishment of the engine on my 400 K4 and today it was time to start. I filled up with coolant, but began to fill the expansion tank, because I felt it was easier to refill there. But the tank become quickly full and then I tried to fill in the "big hole". Started the engine and heated it up for about 30 minutes. Opened the air bleeding but the coolant level does not sink into the exp container. What could it be? I have recycled the coolant I drained, so I have control over how much liquid I filled up.