Also be sure and "tap" the can as close to the top as possible. You want gas to fill the low side only. Liquid filling is way faster but I would leave that to the commercial shops. Interesting but I have had no flow restrictions using the newer style cans?
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Like Karl, I have had trouble with the self-sealing cans and the Mastercool valves. I often have to back the valve off from "full open" to get any flow at all, and now my second Mastercool valve is leaking from around the knob stem. Frustrating. I'm going to have to try Karl's new tool because I don't like buying refrigerant in 30-lb cans when 12 oz cans are only slightly more expensive per ounce of refrigerant.