They are designed to be serviced
Our 1989 has a few valves, and one stuck open, the manual indicates how to open and clean them, simple task.
On yours it may be same valve, it can be services without removing it from the system.
Look for a clip holding the coil on the top, remove it and be careful as they tend to shoot off, tape seems to help.
Next remove the screws holding the plate and post, clean everything out and then insure the spring is good, a little stretch may help.
Put it back together and try it out, wait on the clip until you are completey finished.
The water pressure on the inlet side holds the valve closed, the unit works vai a vent hole, so very little pressure on the spring is needed, but it must be perfectly clean, a little crud or sand and it will not close