Simon.
It was late yesterday I understand your confusion
. Below are pictures which possibly explain the system. The exhaust manifold has two types of cooling water bends crossing the flanges, upper and lower. Upper bends have a small ventingpipe, yellow, may be 20 mm. As all ventingpipes are placed on the highest point in the system and goes to a drainfunnel, you don`t introduce the air into the system again. The lower pipebends have drains on the lowest point, violet, these small pipes may be 30mm, join the header pipe,red, before they merge the main coolingoutlet of the exhaustmanifold and then eventually overboard. I have not been able to trace all details so the correction is based on common engineering.
Tore