Walter -

Could you modify a check valve to flow fast in one direction and slow in the
other?  This would be done by drilling a hole thru the metal plug which stops
the flow.  You'd get full flow in the normal direction, but when flowing
opposite, the internal plug would be situated so as to block the flow but
wouldn't fully seal, since the hole you drilled thru it would allow the flow
rate you want.

I will admit that I know nothing about the L-IV flap system.  However, I have
a quick disconnect on my garden hose that could be modified in this manner to
allow fast flow one way and slow flow in the other.  I don't know if real
check valves are configured in such a way as to allow this.

- Rob Wolf