leaking sprinkler control valve

I am extending my irrigation system and have built a 3 valve manifold system using Toro 1" solonoid controllers. All the valves work well when I turn them on. I've connected the middle valve ( 2 ) to an irrigation system and in testing it found that valve to the left "leaks" only when #2 is turned on. It does not leak when #2 is off. Also I have only wired valve #2 so valves #1 and #3 are not connected to electricity. Any ideas why valve three would be leaking only when valve 2 is activated? Thanks JD
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Just a thought- some things do leak more at low pressure than at high.. Possibly some dirt got in the valve, but it is forced closed at full pressure. Leaks, however, when the other valve relieves some of that pressure. I understand this is solenoid valve, but water pressure may contribute to the seal.
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