I've always meant to show you guys this but forget unless I'm charging a can. The fellow who helps me out on the ranch is a retired heavy equipment mechanic and he had a fit one day when I tossed a spray can of something we were using and I now never throw away a can unless it is absolutely empty.
You need a tire valve stem and you will end up cutting most of the plastic/rubber off the base of it. This is the one that he made for me, he cut the base off with his knife and then I used a wheel grinder to trim it up. You will be removing enough of the base so that the brass ring in the stem can seat down on the stem of the can without the top of the can interfering.
With the core removed, you place the valve stem over the stem on the can. The brass ring will engage the stem on the can and push it down. In that the can is empty nothing happens and if there is a bit of pressure what remains of the fluid might discharge. With the latter in mind, I lightly apply the stem to the can and then push it down on the can with the pressurized air chuck.
I'm solo trying to do this but someday will get some pictures when I have another in my shop. Anyway you cannot physically overcharge a can because you and the stem cannot contain the pressure. You will also hear it charge. My pressure lines are maintained at 100 PSI and in one contact the can is ready to go. I doubt I could hold the chuck or stem on the can for 2 seconds.
It is not that most of the stuff I use from cans is expensive, it just always seems to run out when I need it most. Some of the gun stuff in cans is expensive as is some of the specialty stuff that I have for the farm equipment and I've always hated tossing it when I can feel fluid left inside the can.
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