well...the reality is that the reservoir is acting as a flow restrictor by dropping the wind from the static input of ten inch pressure down to the regulated pressure of five inches. somewhere there is a gate of some sort that limits the flow of air, but more importantly maintains a >constant< pressure of wind supplied to the chest of the trombone pipes. Simply dropping the pressure when the wind is "at rest" (that is, no pipes playing) could probably be done...BUT when demand is placed on the wind by playing one of the pipes, the wind still needs to be a constant pressure....simple answer - rebuild the reservoir (regulator) so that it works properly....why re-invent the wheel?.
of course, you MIGHT be able to use a schwimmer to regulate the wind, but again the point is to have constant pressure whether or not there is a demand being placed on the supply of wind.
FWIW, most reed pipes have a 'window' inside of which they will operate correctly, too little or too much pressure, and they will change character, become slow, might rattle or need to be tuned sharp till they choke...so your window is about 15% give or take (4 3/4 to about 5 1/4 would be about the limits of what the reeds fins "acceptable"...and they do consume a fair amount of cfm's (tho not as much as a flue pipe of the same pitch).
Rick in VA