How does one test the anti drain back valve in the oil stand pipe? I blow in the thing, and cant seem to make the valve close.

FOr that matter, Why is it necessary to have an anti drain back valve in a tall oil pump standpipe? I would think that the oil level in the filter can only drain down to the level of the top of the standpipe, and that there would be no way for a suction to be able to pull any more oil down the pipe.
Am I completely missing something? Yes, if you are using the tall ph8a or FL 1a filters, about a third of the oild drains out of the filter. But that's not critical, or is it? Peace. Yes, SS Dan, I read your posts, and thanks for tips.

Try pushing oil through it.
Or
It might be broken and you've found that it is with your test.
Or
Does your current oil filter already include an anti drain back valve?
Or
Blow in it the other way.

Some info I googled on anti-drain back valves "This is done to avoid a delay in oil pressure buildup once the system is restarted; without an anti-drainback valve, pressurized oil would have to fill the filter before travelling onward to the engine's working parts. This situation can cause premature wear of moving parts due to initial lack of oil."