I put together the filters today, two 3" bulkheads to Join Barrel A to
Barrel B and two 2" bulkheads to drain Barrel A and Barrel B. I still have
to put the 3" inflow bulkhead in Barrel A.

When I first started filling, only the 2"bulkhead in barrel A leaked, then
the 2 3" bulkheads started to drip. I suspect the weight of the water in the
barrels put some pressure on the 3" bulkheads. The 3" bulkheads almost
stopped leaking, but the one 2" bulkhead has a steady stream.

I think I'll disassemble the 2" bulkhead, as there almost seems to be a
physical defect (trapped plastic or cracked gasket), but what's the best
overall approach? Silicon on the Flange/Gasket side or Silicon on the
outside? All the bulkheads are nut in. The 2" bulkheads probably need
tightened a bit more, but the nuts are a little thin.