Mark replied:

Hey Fred, what do you mean by breather valve? You need a low point in it (like a 90 deg junction with a ball valve screwed into a screwed cap) to act as an outlet and a high point (like a 90 deg elbow with screwed cap) to use as a fill point. 100mm would be a good size. Visit you local plumbing shop and let them know what you want. I'm sure they'll help you out.

brenton replied:

gday fred ive used abs pipe 160mm dia it is 10mm thick walled structualy stronger than pvc drill and tap the end to take galv end plates 25mm hose for the filler 6mm plastic tube breather goes up inside the body work

brian replied:

hi fred if you have a trayback, consider a small caravan/camper water tank. i have placed a 60lt rectangular tank between the chassis rails quite easily. just a choice