I had the same problem and it seems to be quite common. Firstly, you must stop the leaking valve. Try lapping it with toothpaste with very light pressure. Grinding paste is too course.
I managed to find two small O rings from a local bearing shop. Measure the shaft and go half mm smaller, iirc 3mm. Slip them on the end of the solenoid shaft with a little rubber grease between the O rings and the solenoid body. When the solenoid is not powered and the shaft retracted the O rings should 'butt up' nicely to the solenoid.
In our climates, the solenoid should only be powered for a few seconds. I drove around for a few days with the solenoid unpowered in order to confirm the valve had stopped leaking.
Regards
Chris
Cape Town