A trip to bunnings resulted in this contraption.
My blueseas water maker does not like getting air in the system and even though the salt water intake is low in the hull, any air that is draw into the system accumulates in places like the pre-filters and eventually stops the main pressure pump from working. Easily fixed with a quick back flush of fresh water, but I thought "Wouldn't it be great if it never happened"
Water comes in the centre connection of my new fan-dangled device and out the bottom to the pre-filters. Any air goes out the top and there is a ping pong ball trapped inside the top section that acts as a valve. There is a small priming pump (came with the w/maker install kit) that feeds the centre section. Works like a treat.
Well it worked like a treat after I put small holes in the ping pong ball so that it would fill with water and become heavier. The problem was that it sealed so well, that the air pressure was able to hold the ping pong ball at the top of the contraption and not let the air bleed out.
Took it for a test on the weekend and I could actually see the odd bit of air being released form the system as the discharge is into the galley sink.
If you want to make one, it is from pressure pipe and just take a ping pong ball with you and find the pipe that it just fits inside. There is not much clearance. I had to glue a bit of pipe inside the "T" joint to the ping pong ball could not drop to the lower section. (this would not be good) The bigger the lower section of the contraption is the better, as it gives you a bit more surge capacity if it gets a gut full of air.
The ping pong ball valve idea came for those snorkels we had as kids. (well some may be old enough to remember them) 
Hello HG, I actually brought one of the flubbed units to see if I could use it on the boat some how. I had thought I might put it into use when anchored up. Or even try and make my own using their design. But in the end I decided it was too big to try and store on the boat and the amount of fresh water it produced would probably only be suitable for emergency situations. Sold it on gumtree in Feb this year. The guy who brought it wanted to try and set it up on a mates catamaran. Maybe he did.
The unit I brought was their smallest units at 1150x1150 x50mm deep and would provide 5 ltrs per day.
The other link to the water maker looks promising. Mine is a lot physically bigger and has something like a 5 year warranty on the pump. It is rated at 40 LPH but easily put out 55LPH. The only issue is that it is power hungry (30A motor) and you have to run it for about 15 minutes at the start to flush the old salt water out before making product for the tank.
I installed it on the boat about 3 years back and have never put any other water in the tank. Tastes good and you know it's safe to drink. Been to places like New Cal and Vanuatu and it certainly gives you one less think to work about.
I normally run the diesel (usually also moving anchorage and charging batteries) when making water so 2L of diesel equates to 55 Litres of water. Makes more sense then carrying a 1000L of water on an extended trip when you can carry an extra 18L of diesel to get the same result.
The water makers aren't cheap but it would have to have been a better investment than my wind turbine which needed a Targa to mount it on.
Scaramouch has given a fridge compressor mount to suit my Bukh DV24 and Ive contemplated mounting a ceramic high pressure pump maybe from Gerni and add to the build up from there adding a a/c pulley clutch and sorting it out slowly Id be happy with a 20 litre out come I may have to run a third pulley as the flywheel pulley I have fitted I think will be to slow . That's next years hope and dream
The ping pong ball valve idea came for those snorkels we had as kids. (well some may be old enough to remember them)
OOh yes I remember them, Lucky any of us survived!!! 
Here is a link for the manual for mine. May help with sorting out what you need for yours. I've heard of plenty of people building their own. Cheers
www.echotecwatermakers.com/pdf/pamphlet/DML-200-260-Manual.pdf#zoom=100
