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 …
The Smell of Molten Projects in the Morning 
If it’s just the siphon of the tube from the water pump, you could break that with a t-connector and a small tube going up from there.
I think breaking the siphon would leave the icemaker reservoir full while draining the rest of the plumbing. Turns out draining the reservoir is no problem, but draining the laser tube’s water jacket produces a mighty bubbling noise while it refills, then takes a few more minutes to get the last of the bubbles out of the jacket.
It surely doesn’t matter either way; I’m just being fussy.
Draining everything after use might even be a clever design feature. As The Doctor said: “Water is patient […] water just waits”.
Safer to just get it out of there.
But then it’s all together. Who knows what goes on in that bucket?
Surely it’s plotting and scheming for the Great Breakout! [sigh]
I think you need a strainer more than a filter in this case.
Good point.
I must pull the pump to see if it has a hose inlet fitting letting it run inline, rather than submerged, whereupon a cheesecloth ball on the inlet hose should snag the bigger chunks.
Then I’ll discover its power dissipation requires submerged operation, even if the fitting suggests otherwise.
Doesn’t running the tube at 0C result in enormous amounts of condensation all over your everything, including your HV wiring??
Because the icemaker drops nine little ice bullets into the reservoir every eight minutes, after a few hours it can cool five gallons of circulating water from 20 °C (ambient basement) to 14 °C. That’s with the laser tube inactive: if I was actually cutting, the water would stay warmer.
The humidity is around 60%, so the dew point is under 10 °C. If I ever get the pump out of the bucket under the icemaker, then the water temperature might gradually get low enough to be worrisome; obviously, I must start cutting more stuff!