Sherman fuel tank overflowing

[Malcolm Towrie]

We have a puzzling problem with the left hand fuel tank on one of our Shermans overflowing after being driven for a few miles. These are twin diesel M4A2's.

The fuel system is not stock. At some time in the past, it was converted to the two tanks on each side supplying only the engine on their respective side. The supply tubing goes from the bottom of the lower tank to the blower-driven fuel pump (vane type) to a single fuel filter to the injection gallery on the cylinder head.

Return flow is from the return gallery on the cylinder head to high on the upper tank.

Also, there is an interconnecting balance tube running from the bottom of one lower tank to the other to keep the tanks at the same level.

This problem showed up when we filled the fuel tanks on both sides right up.

After driving the tank for a few miles at fairly high speeds to an event, the left hand tank started overflowing badly. It turned out the fuel cap did not seal, so it was switched for the good cap on the other side. This stopped the leakage but the left tank was still found full to the brim after the drive home.

We figured the hose that connects the upper tank to the lower tank was plugged and the return flow to the upper tank could not drain down to the lower one which supplies the fuel. But pulling the hose off showed no signs of blockage either in the hose or in either tank.
That's where it stands. Anyone any idea what could be going on?

Malcolm