After my posting yesterday about a check valve that would allow full flow in
one direction but reduced flow in the other (to enable IV-P flaps to come up
slowly) I received the following response from Harold Weathersbee.  It sounds
like a much better solution than my lame suggestion, so I reprint it here. --
Rob Wolf

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Try W.W.Grainger.  They stock check valves that are free flow one direction
and adjustable flow in the other direction.  I have used them in many
aplications and they work great.

Harold Weathersbee
IVP 85% N68WB