I did a crude experiment with my Andair bleed-option (CK250E-B) check valves and found that the wrong-way flow is about 1 gallon/minute at (whatever I can manage with my lungs) about 1 psi.

So the first question is how much gas will they flow in the wrong direction if they flow 1 gallon/minute of air.   ?

The second question I posed was this:   Suppose you have only 10 gallons in one tank and you take off and climb to fl200 in x minutes.   Up there the pressure is half what it is on the ground so you will have to vent 40 gallons.   So if x is less than 40 you are going to have a positive pressure in the tank when you arrive at fl200.   In my plane, x could be as low as 10, leaving multiple psi in the tank.   This doesn't seem healthy.

So question for anyone who has these valves installed:   1) if you've ever done something like my proposed scenario, did you observe the wings bulging?   2) are you convinced there is any benefit to the valves so far as reducing fuel going overboard on the ground?

I did disassemble the valves and determined that I could modify the outflow rate by drilling out the two little holes in the shuttle.  However, this would further diminish the benefit as far as retaining fuel.

Overall, my conclusion is that the valves as shipped have inadequate outflow rate which will create excess pressure in the wing if you climb with partially filled tanks.

Colyn