How can I keep fuel from escaping via the fuel breather hose /roll over valve? If I fill the tank the vapours build pressure and push fuel up the hose. Its a standard Westy tank, welded in valve in the top of the tank, 6mm fuel hose going vertical out the tank then turns 180° back to the floor.
Brand new valve and hose.
If I leave car stationary in the sun then remove cap vapour bursts out. So it's not venting. Hmmm wonder if the valve isn't actually at the highest point. Maybe in the top of the tank but if the tank is tilted on mounts forward (the valve is top front of tank) the valve will be below fuel surface during full tank.. I'll look at that, maybe able to tilt tank backwards on it mounts.. If that's the issue.
Anyone else had similar problems with fuel vents?

What valve have you used? Lots of people fit a one way valve that lets air in but prevents gas out. You need a two way breather that lets air in gas out but not fuel out. Also fitting them remote from the tank (rather than direct to a boss) can help prevent them shutting off due to fuel surge and staying shut.

I tried several cheap (wrong) ones before I bit the bullet and fitted an expensive one. No issues since.

https://www.merlinmotorsport.co.uk/p/roll-over-valve-in-line-with-5-16-8mm-push-on-tails-m-trl88

https://www.merlinmotorsport.co.uk/p/roll-over-valve-6jic-inlet-x-1-4-outlet-m-trl8

That's the exact one I've got fitted, yes arrow right way.. 👍

Then it may be that when full, the fuel level is shutting it off or fuel surge doing the same and not draining back to let it reopen. If you could mount it remotely with hose between tank and valve might help. You'd need to ensure the valve is higher than tank and hose has no dips in it. Mounting the valve at the opposite end of the tank than the outlet will reduce fuel surge (if outlet is at one end)into it. If outlet is central keep it as high as possible but still central.

The valve is mounted in the Breather hose not the tank.
Maybe extending the length of the Breather hose to make it higher will help.