Sprinkler Valve Question

Hi. I'm having a problem with the sprinkler valve on my next door neighbor's system. It's an Irritrol 2711DPR valve...and it works fine loosening the "manual" screw... but when using the controller, the solenoid hums, a momentary gush of water comes out the sprinklers, then cuts back to just a trickle. All the rest of the sprinklers in all other zones work fine. Voltage is the same as at the other valves...26.3 vac. Could this be dirt in the valve? If so, why would it not affect performance on "manual"? If it is dirt, are Irritrol valves easy to disassemble?...and anybody have any idea what a small tab on the valve marked "<on-off>" does? Thanks in advance.
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