stafford wroteI take it then it's just a gravity fill through the header tank.
Yes, not rocket science.
P.S. Don't mean to be flippant! Just take your time, fill it a little at a time until the air can escape. Probably takes a half hour to do it if you want to get all of the air out. (maybe less, drink a beer or two as you do it and it seems to take less time :-))
