1998 Duetto fresh water tank breather venting

I have just removed the water tank to try and solve this problem.
Once again I am amazed at the costcutting  from Autosleepers. The tank has 4 bungs in the top with small (badly drilled) holes in them , rather than avent pipe taken to a level above the filler cap level.
This means that when the tank is not completely horizontal it will lose water , at 10 degrees it will lose about half. , easily achieved whilst in motion.
Also the water level sensor is positioned at the very rear of the tank , if positioned centrally it would enable the level to be determined more accurately than it's present position when not level.
The design is fine if the van is parked up on a flat surface , but it is less than perfect for a camper.

I can't say i'm surprised. A/S aren't the first and won't be the only manufacturers taking cost cutting measures. It happens with all and i once remember reading an article about Hymer vans where older models were actually commanding higher prices than later models......simply due to sacrificing build quality on later models to keep the price competitive. 

The tank float position as you say is a bit daft and really not properly thought out, but there are many strange things similar to this which A/S did. For example....a split shower tray requires removal of the entire toilet....simply because part of the tray sits under the toilet! Turns an otherwise relatively easy DIY job into a major job, or if having it done professionally....what would have been an inexpensive job, expensive!

Your 16 year old Duetto was originally fitted with nylon plungers in each of the holes that shut the hole when water surged against them.  One of them on my '99 Duetto disappeared when the van was 5 yrs old and caused water to surge out when braking.  The Dealer and AS gave all sorts of theories as to what the problem could be and it was only when the AS service centre technician dropped the tank and showed me the set up that I understood.

The service centre blocked the four original vents and created a new vent pipe from the top of the tank to a venting aperture that already existed on the water filling point.  An excellent modification that worked perfectly.  I would suggest you do the same.  Alternatively, having blocked the existing vents connect a length of rubber hose to a new vent and twist it into a 'swan neck" held in place by a 'p' clip screwed to the bottom of the floor.

Peter

the tank on my Duetto had 4 bungs with small holes drilled in them , as I said before , no complicated plungers.
I have now vented the tank more or less in the manner you suggested, thanks for your interest.