Comment #1: It would be a waste to install a check valve if the only goal is to prevent unfiltered oil from getting back to the sump. Afterall, where is the oil going to go besides the sump? From there it is pumped through the filter anyways. It is going to get filtered. Everything between the filter and sump sees unfiltered oil all the time. The sump is full of oil that has been in the engine and full of the stuff that needs filtering.
Comment #2: Filters in a vertical mount situation (filter opening towards the ground) may indeed pass all of the oil in the fiter into the passages after the filter, leaving the filter itself dry. It just depends on the level of the passages after the filter. Setups like this in all likelyhood never needed the ADBV, but they were used for commonality in a product lineup.
Comment #2: Filters in a vertical mount situation (filter opening towards the ground) may indeed pass all of the oil in the fiter into the passages after the filter, leaving the filter itself dry. It just depends on the level of the passages after the filter. Setups like this in all likelyhood never needed the ADBV, but they were used for commonality in a product lineup.