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.