You can do the job with a 2/2 valve, if you use the Cutlet system. The standard on FP feathers...
The 2/2 valve must have the highest flow you can afford, and a small hole in the rams pressure chamber will dump the gas. Leave the valve open, and the bottle will empty itself trough that small hole (we talk about 1.5-2mm diameter).
If you go LP, affordable 3/2 valves ain't a problem.
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