Hmm, I was thinking a rubber test tube stopper with a hole thru it for the airline tubing might be used in place of your latex glove as a cork for the end of the syringe.
It might work. The difference in my design is that the latex glove collapses, which opens up the syringe part completely. If you use a rubber stopper, there is still the back pressure from the air pump membrane which might prevent the refill.
The potential weakness I can see for this design would be the use of cheap aquarium check valves which tend to eventually fail and get stuck especially when exposed to liquids with stuff dissolved in them.
You are right... that might or might not be an issue over time. Here
they sell check valves with a different design, might be worth to check out.
I'd be curious to know how long it runs for before it is in need of repair.
Me too. It's still in the trial phase. Once I have it running for a few weeks or months I will let you know.