I've completed my marginally scientific tests on the Andair fuel check valves part number ck250E-B.
Summary: bleed rate is too slow to balance the pressure in the tank by about 2:1 in a max rate climb.
If you fixed that the bleed function could still cut your spillage as the result of a taxi turn or a short taxi
across a slanted ramp by a factor of 78:1. Of course it would have no effect on the result of leaving your
airplane parked in the sun or on a slanted ramp.
The introduction of the check valve does increase the risk of blockage in either direction.
bleed rate of air: .82 cups/second.
bleed rate of water: .00159 cups/second
unrestricted flow rate of water: not less than .25 cups/second.
Climb pressurization problem
Then, suppose you climb at 2000 fpm to fl200 and one 51 gallon wing tank is empty. That will take you 10 minutes.
The density of air is about half what it is on the ground so you will have to move a maximum of 51 gallons of air (albeit less dens air as you go).
time to bleed 51 gallons of air: (51*16)/.82 = 995 seconds or 19.9 minutes.
best case climb time to fl200 = 10 minutes.
flow rate must be increased 2x to insure pressure inside the tank is always balanced to ambient.
Fixing it
You can increase the air bleed rate by drilling a couple more small holes in the bleeder side of the mechanism.
This will increase the liquid bleed rate. I think this is a non-linear function but taking the worst case linear result, that would yield .00318 cups/second liquid flow rate.
max water (standing in for fuel) that could be dumped in a 5 second turn = .0159 cups
unrestricted 1.25 cups
max fuel that could be dumped in a 20 second taxi = .0636 cups
unrestricted = 5 cups
Risk of blockage
The check valve mechanism has two extremely small (about 1mm) holes in the bleed direction and 8 approximately 1/8" holes in the normal flow direction. The modification I envision would add two more 1mm holes in the bleed direction. I think there is very little risk of blockage (beyond that of the vent tube itself) in the normal flow direction. In the bleed direction, if your tank had a lot of granules on order 1mm in size, and they managed to find their way to the vent tube, and they all gathered on the bleed outlet at once, you could get blockage in the bleed direction. However, that piece moves when the flow changes back to normal so that blockage of the bleed holes would not affect inward flow.
Conclusion
If you have these valves installed, you might want to consider modification or removal.
If you are considering modifying these valves and adding them to your vent system the tradeoff is as follows:
In exchange for adding a risk of blockage in the bleed direction you reduce taxi spillages from not much to nearly nothing. Spillage due to fuel expansion or parking on a slanted ramp will be about the same with or without the valve installed.