I though this might happen under pressure!
. the lenght of the tube to the Y is acting as a 'buffer', but as the pressure increases it becomes easier to pump back into the tube than to pump through the system
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one way valves on the tubes would work. a ball bearing type thing would work well enough for our needs would'nt it?, a flap would be better cause it'd open up the whole tubes diametre, or the 'ball' valve could be oversized...