I don't think it's the self test as the gauges have changed before I actually start the car. If my main tank is almost empty and the sub half full I can see a drop of around 1/8 of the sub tank in a couple of days (have a work vehicle so Patrice may sit dormant for a couple)
Now you mention it mud runner both tanks make a hiss when the caps are removed but I've never really taken note of whether it's positive pressure or a vacuum, usually to busy staring at my nissanpatrol.com.au sticker.
Please excuse my silly questions but where might I find the breather tubes? Do they come straight off the top of the tanks as I may extend them somewhere a little higher to prevent s#*t getting in.
Will definitely take not of what happens when I remove the caps next time....