Question
When water is turned on, water leaks through the cap.  How do I remove the cap to get at the diaphragm and determine if it is dirty/damaged and in need of replacement?

Answer
Hi Ernie
You can unscrew the bolts around the valve and clean it. If this is an backflow valve it should have two parts the one with bolts and the selenoid and the other without bolts, this part have a vent and it is normal some water comes when operating, but usualy stops. If the water comes all the time probably the cap is cracked, you can buy e new one and replace the cap.
Please note that Rain Jet is now Lawn Genie.
Regards
Guillermo