Originally Posted By: WobblyElvis
More like 0.1 psi at room temperature. Unless my lungs are much stronger than I thought.
Yes, as that implies, on most canister filters you can easily do an oral pressure check, for both forward and reverse (i.e., drain-back) flow. Pressure necessary to open the ADBV flap can vary with the age of the filter. When new, some brands used to require enough pressure to cause a problem with some cars, for example my brother's 1974 VW Dasher. On another hand, the forward opening pressure shouldn't be zero, either.