Water gage conundrum ?

[Jolly Roger]

I have a water gage problem which has been driving me nuts for months, and I wonder if anyone with experience with pneumatic gages can throw some light on it.
I have two 150 gallon water tanks either side of the boat. The tank gages are air pressure operated, like the Tank Tender system, but older, circa 1977. The makers are long gone.
A simple 3/8” straight plastic tube, the depth of the tank, bolts into the top of the tank with a gasket and six screws. A 1/8” thin plastic pipe pushes in the top and goes to the gage where it pushes into a tube on the gage. The gages are made like a pocket watch, having a diaphragm and gears which activates the gage indicator as the water pressure varies.
One side works perfectly—when I fill up the tank the gage moves to full and decreases as the water is used. When the tank is empty the gage shows zero.
But when I drain and fill the tank on the other side the gage slowly moves to full, then drops off to zero within about an hour, but that varies a bit.
"Air leak" you say! Either in the plastic pipe or gage! but wait. I’ve installed new connecting pipe, which I pre-tested for cracks or leaks. I’ve also sealed all the joints in the tank tube with epoxy, including the connecting pipe joint.
Here’s the rub. If I swap the gages over, the good gage drops off to zero and the faulty side gage stays at full. So it’s not a faulty gage, right?
Also, when I submerge the faulty tube in the bath-tub at my home it records full, (with either gage), and stays full, the last test being a week in duration. There is absolutely no sign of air leaks in either the tank tube or the connecting pipe to the gage.
But the moment I screw it back in the top of the full tank, bingo! it drops off to zero within an hour, with either gage fitted .
I don’t want to change the system for a sight-glass or lever gage, because it doesn’t use any power and is really simple. One side works perfectly, so why doesn’t the other?
These sort of conundrums are frustrating, like intermittent electrical problems, so any thoughts would be much appreciated.