I have a 3/4 inch inline automatic sprinkler valve that will not shut off, whether it is turned on electrically or by unscrewing the solenoid. It drives a drip line with about 15 heads, so the flow rate is rather low. If I close the main shutoff valve to the system and reopen it or open a drain valve downstream so the flow increases dramatically it closes fine, so it appears to depend on the downstream pressure being high. Can anyone help with the cause and fix? Normally I would just replace the valve as they are not expensive enough to fight with, but I am concerned about getting a new one in. I have unions both sides of the valve, but the pipes seem fixed in the ground. I would have to find one the same length and get the unions to mate without pulling or pushing the pipe.
Opening a drain valve downstream will not raise downstream pressure, it will increase flow and lower total system pressure.
If there is any flow the valve should close, as pressure above the diaphragm equalizes with supply pressure upon closing of the solenoid pilot port.
Get the brand and model # of the valve and order a rebuild kit for it as your problem is very likely to be mechanical. Turn off the supply and remove the valve bonnet, inspect and clean internal components (flush all ports to remove debris, check the diaphragm and support rings for damage or cracks, inspect the sealing surfaces and seats). Replace the washers and diaphragm with new ones from the rebuild kit.