Had a look on the net for these devices. Looks like its a tube about 40mm dia. and tapers at each end to 16mm. Inside is a ball which should most of the time sit in the waist of the unit allowing liquid or air gas to move through the device freely. Any rush of fuel, water or air will move the ball to the end against the tapers and seal off the tube. So to work this device needs to be installed so it sits horizontally at all times, otherwise it can block of the fuel tank with interesting results. It will have to be installed east/west on the boat and on the horizontal plane. I think its a bit risky, if your boat is loaded so its leaning to one side, it could activate.
When do you refuel, with the boat on the trailer, and or while afloat? if on the trailer is the boat sitting bow down. I.e changing the hitch height may make a difference.

Yea I think the idea is that they actually sit vertically. The idea being that if its only air the ball will rise (but not to the top) and allow air past and around it. In the event its a liquid the ball will float all the way up and the valve will close.

To be honest i'm thinking of coming up with an alternative to these units. Whilst they may be ok for the occasional surge they leave no wiggle room for a true air lock situation. i.e. if fuel NEEDS to escape in order to vent air these units will not allow it..

I'm thinking of doing a bit more research or potentially seeing if I can get a barb fitting that will increase to a larger diameter line, running a about 200-300mm of larger line, then back down to original, this would act as a mini reservoir and hopefully minimise the chance of fuel coming out the breathers. If fuel went in it can get back out, and if fuel NEEDS to escape then it still can.

Still keeping my mind open at this stage

And iana, it sits pretty level to be honest. However, the two breathers come off from the bow end of the tank, one from port and one from starboard but the filler enters from the stern end of the tank on the port side. Oh and they don't come off from the 'top' off the tank, they come off from pretty much as high as possible on the sides. If that makes sense. This is due to limited room under the floor

Thanks

I think you have to set the boat up on the trailer so it is bow high, so that the floor slopes up and the breathers are clear of liquid while filling. You could as an experiment at the fuel station leave your trailer chained and electrically coupled, but lift the hitch and using the jockey wheel raise the bow. Perhaps put a spirit level on the floor to see the angle. Then fill and see if there is an improvement.
While there is fuel blocking your breather line while filling I think you will have a problem.

Yea I see what you mean. I'll get the level out and have a look next time I get a chance. Thanks for that

Thought I'd update this.

So today I did as you suggested Iana and filling was ALOT easier!! The pump actually auto clicked off and only a minimal amount came out one of the breathers. Certainly the quickest fill ever.

Thanks Mate!

Really happy about having helped someone on the forum, however even happier that you've taken the time to give an up-date. I wish some of the other members would respond to help and effort put in by others. Yes some do but a lot don't.

Too easy! You took the time to help so its the least I could do.