ooooooooooooooooooh also, one thing to check that ive came accross a couple of times

on the back of the head, theres a small pipe that comes out of the head (g/box side)

the aluminium 90degree pipe currodes and blocks solid with currosion and shyte, make sure the pipe is free and the hole in the head is too!

this is the "bleed pipe" that returns water/air to the tip of the header tank