Uneven fuel level in tanks.

[Aussie Steve]

My C120 used to have taps on the manifold above the gearbox. A long time ago I had trouble with it only drawing from one tank and loosing its prime overnight. The taps had screens on the inlet from memory. I priced new taps from Freightliner and decided to remove the taps all together as they were worn on the tap shaft and sucking small amounts of air. It took me a while to troubleshoot the fault. So I removed the taps and plumbed everything together, it now draws and returns to all four tanks evenly. For those who will say what if...... and you need to isolate a tank for some reason. Well in eight years since this mod the need has not arisen yet but in the toolbox is enough JIC plugs and caps to isolate three tanks, this truck spends 95 percent of its time off road in pretty ordinary conditions.
The taps would be a good starting point, Oh and it's Cat powered but I don't think that would really make a difference.