Because the icemaker sits atop the cooling water bucket, when the pump turns off the water drains back through the laser tube into the bucket:
The bucket contained all the water to start with, so with the icemaker and laser tube empty, all the water is back in the bucket. Getting all the bubbles out of the laser tube takes a while after the pump starts running, so I stuck a check valve on the laser output tube in the icemaker’s reservoir:
Which, after a few days, developed a slow leak, once again emptying the reservoir.
There being no way to dismantle the valve for analysis and cleaning, I just cut it apart:
Lo and behold, a small tangle of thin fibers had found its way into the valve:
Which held the silicone disk ajar and let the water slowly leak backwards through the valve.
I have no idea where it might have come from, but a simple filter seems like a good idea. Given that the pump produces pretty nearly zero pressure, anything fancier than a coffee filter in a funnel would present too much back pressure.
Or, with three more valves in the bag, I can wait to see how long it takes for another tangle to arrive …