Good idea for an checkvalve for around $5 in parts for an air compressor. I have looked at getting some check valves myself and they are pricey ($20 and up). So this is a nice alternative. On comment asks about using this with water, which I guess is possible byt I would wonder how long everything would hold up to the corruption water could cause.
http://www.instructables.com/id/Inline-Check-Valve/?ALLSTEPS