Here's one I made for a bucket setup. One of the first things that you will need to do is to make sure that the surface that the o-ring is going to rest on is smooth. You can take a small dowel that is as close a perfect fit inside the hole as possible, and put some grinding compound or very fine wet sand paper on the end of it and polish the surface that the oring is going to sit on.
Some parts that I have came smooth enough to work, and others didn't. If it doesn't seal it won't work right.
Here are the parts I used. The tiny o-rings are from re-build kits you can get for smart part on-off valves. They fit just right and are a high enough durometer to work for a check valve.
I got the ball bearings and compression springs at a Sears Hardware. You need them to be a very close fit. I took the base of the bucket changer in when I bought the parts.
Next drop the o-ring in.
Followed by the ball bearing.
The last thing is the spring. You will have to play around with the length of the spring until you find the right size. You may need to cut it. You may need to bend the end of the spring on both ends, so it doesn't creep over the ball bearing and jamb things up. Or get snagged on the the pin depressor on the ASA end.
If it's too strong, it won't let the CO2 in. If it's to short, it won't stop the CO2 in the valve from leaking back.
It really only serves to hold the ball bearing in place on the o-ring. The check valve uses the pressure of the CO2 in the valve to hold it closed. That's not the springs job. The spring should not interfere with the valve being able to recharge. If it does, you will lose shots per 12 gram, or it won't work at all.
Get a couple of the springs. They are cheap, and you will probably go through a couple until you get it right. At least I did.
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