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Why would 2 out of 3 sprinkler zones cause the pressure reading at the pump to drop to 0? This is a well irrigation system and it is separate from the home's well. The pump is a 3/4 HP, has a flow rate of 10 GPM and is designed to maintain pressure at 40 PSI. The pressure tank is 2.1 gallons. The pump turns on at 30 PSI and off at 50 PSI. Zone 3 maintains 40 PSI perfectly, but zones 1 and 2 immediately drop to 0 PSI when activated, occasionally creeping up to 10 PSI but no more than that. There is still flow to the heads but they don't pop up. Is it possible there is a leak in both zones? If so, why would the symptoms be identical? Surely it would vary to some degree? I see no sign of leakage either, even after capping off all the heads. With capped heads, all symptoms are the same (0 PSI reading at the pump with an occasional 10 PSI). Assuming there is no leak, why would the pressure not build up with capped heads? If one zone is working fine, it couldn't be the pressure tank or pump, could it? The only thing I can think of is that there is a large obstruction in the mainline before the bad zones but after the good one, but even then, why would it not build pressure (since it's an obstruction)? The water must be going somewhere. Any help appreciated.

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Either you've got so many sprinklers on that line that the pump isn't keeping up with them (unlikely given that you say you tried capping them), or water isn't getting to your pressure gauge, or there's some outlet you haven't accounted for such as a leak. –  keshlam Jun 4 at 16:10
    
@keshlam Well every single head is capped and the system still won't built pressure. The pressure gauge, which is brand new, is right off the pump, and it works because zone 3 builds pressure just fine and maintains 40 PSI. All this makes me think there is a bad leak somewhere that I'm not seeing, but zones 1 and 2 are the same with 0 - 10 PSI on the gauge, so that would mean there are 2 different leaks that aren't visible. It just doesn't seem likely. I'm stumped on this one. –  PJS1987 Jun 4 at 23:53
    
I don't have any other answer, assuming you're operating the system correctly as you have described and the pump isn't simply being intermittent. –  keshlam Jun 5 at 1:34
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