There's an AD addressing the requirement for the vented caps on
Cessnas (79-10-14R1), and this refers to Service letter SE77-6, which
gives the cap part numbers. The R182 manual I have here also gives the
testing details for these caps, which requires that the check valve
open at 4 inches water column or less of negative pressure in the tank,
and it must not allow any leakage out of the tank at 0.7 PSI or less.
It stipulates that any fuel staining around the cap is cause for
investigation.
Please tell me, Jim, what's wrong with insisting that the caps
aren't supposed to allow air out of the tank (except at high pressures
as a safety release) and thereby upset the pressure balance between
them and allow fuel to cross over in flight, which often leads to fuel
loss in flight?

Dan