Can you get a siphon started at all? If you can, and it fizzles out after a bit, then chances are it's sucking air somewhere in the siphon loop. Either from the tank inlet, or the pipe connections. If the vent hole is not large enough, it may pull so hard it pulls a vortex in the tank end and sucks air that way?
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Dan Pesonen
Umm, a tank or 5
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