every setup i've ever seen like that that has the regulator with the VFF30 filter lockoff has always had a steel line connecting the two. And the size of the line is usually 3/8 size tubing.
Even though copper may work it will probably cause some issues down the road like the line breaking and the size of it also may cause issues down the road. Only time will tell but if the mfg designed it with 3/8 steel line i most certainly would put that back on there.
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