Perfect!! Lol no i am having sortof the same problem but not really. First of what i recommend is to get that solenoid off and but plumbers epoxy in the hole. It hardens like steel. Give that some time to harden and you should be good. But something that might be more of a problem is the size of your sprinkler valve. Is it 3/4 inch? Well because i used a 3/4 inch sprinkler valve on my first cannon and i found out that since sprinkler valves have less flow that a ball valve but open quicker than a ball valve, i found that i had not so good performance because it had little flow. In this case i found it that a 3/4 inch ball valve had better performance than a sprinkler valve only because if the fact that it had a larger flow. So maybe try that
EDIT: Woah, Woah, Woah, Just went right over my head.

That i believe is a DWV fitting on the end of your cannon. And are the other fittings also DWV, cause i cant see. That is dangerous and will have to be fixed if not rebuilt.
Now for my little problem
I have a 3/4 inch sprinkler valve that has been moded with a blowgun valve. Now since this valve had crappy performance on my first cannon, i decided to mount it onto my newly created 1.5 inch piston cannon which you can see
here. I just put it on my cannon because my blowgun didnt have enough flow to actuate the piston, and i would prefer to use a trigger actuation rather than a ball valve. So i got the fitting and put it on and tried it out. Now i noticed that it honks even though it has been modded. So can anyone help

If you need a pic just request it.