Try the diaphragm first....BUT....what I would do is go buy a new valve, install a new bonnet(plastic top part), new diaphragm and new solenoid....
The entire valve may cost less than buying all the parts seperately, and sometimes you will find that you need the other parts anyway.
Good luck and don't forget not to pinch the nipple! (sprinkler tech joke, be careful not to pinch the little "nipple" on the bottom of the bonnet that is directly under the solenoid....)
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