One zone lost pressure/fails to come on. Hardie solenoid valve. Swapped solenoids (same result). Open bleed screw → full pressure; close bleed → pressure drops to nothing. Solenoid hums but no water.
"Chances are that you have a problem with the diaphragm inside the valve. These are a little tricky to replace, but can be done with care. Also clean out the ports (the little holes) in the valve body. If this doesn't work, then you will probably need to replace the valve."
The bleed screw test is diagnostic: opening it relieves pressure above the diaphragm, allowing the valve to open. If it closes when bleed is closed, the diaphragm isn't sealing or the ports are clogged, preventing pressure equalization for closure.
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