I'm not sure if this will help but baseboard heating systems use pressure reducing valves that automatically refill the system when the pressure gets too low. Here is a link for one that goes from 10-25PSI. Best of Luck.
http://www.supplyhouse.com/Watts-0386450-S1156F-1-2-Union-Solder-Iron-Pressure-Regulator-3572000-p