A high pressure air chamber is easily filled by a higher pressure air pump attached to a valve at the highest point of the chamber. Excess water would either go out the way it came in, or as for common compressor pressure tanks, a simple bleed valve at the bottom lets the water out. Close it when it discharges air.For air, a simple spring plug valve similar to those of car tires are common. For the water discharge, again a plug valve with screw in movement is common. Those are what came installed on our water pressure tanks and compressors.For bigger industrial applications, bigger and more robust valves would be used, with common automation sensors and actuators.