The the engine began stalling on the rear tank at progressively higher indicated fuel levels. At first, I thought it was a
level sender problem, but refilling the tank didn't always help. So eventually, I
pulled the bed and looked inside the tank. There was a LOT of fine muddy sand in the bottom - probably due to the degraded rollover valve grommet allowing it to get in from the top of the tank. There was so much that it was inside
the FDM, and even in the pump motor. It was beginning to block up the fuel filter (which I cut open to inspect).
IF THE IMAGE IS TOO SMALL, click it.
But in addition to cleaning most of the sand out of the tank and FDM reservoir, I also hung the pump motor (bare) by some wires in a gas container to flush it out, and it cleared up quickly, with apparently good flow. So I re-assembled the FDM and found that the pump still made good pressure. Despite several laughs by my mechanic friends, I put everything back like it was (minus MOST of the sand) with a new filter, and the truck runs fine.
UPDATE: That was Oct.'22, and the rear tank has been working for nearly 2 years without problems.
Change the old bed bolts to the new style that's better:
