i dont have that type but i would just fill both sides with watater and stick a small hose into the u tube and suck out the air i think the water will follow
use small airline tubing, push it to the crest of the bend (top) and suck the air out. This will start the siphon, and any leftover air will be sucked out of the tube when the flow starts going.
just be sure not to get water in your mouth...lol it doesnt taste to good lol hubby was sucking the hose to the protein skimmer and got water in his mouth, needless to say he spent some time in the bathroom after that..lol good luck
if you have a problem with the siphopn brealing which can happen when air bubbles start to form in the top of the u-tube. there is a flat sided
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that you can drill a hole in the top of the u tube silicon in the
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attach an air line from it to a venture valve on a powerhead it will suck the bubbles out before they can cause a break in the siphon and cause a ton of problems.
HTH
Mike