Simple procedure, push valve to remain fully opened and squirt compress air inside valve, if with sand, debris, air should clear it, if not, with valve tool remove air valves from tubes. Don't lose lower valve assy inside tubes when removing upper assy. Once out push pin fully in to remain opened and with brush and soappy water clean the lower rubber seal and lower valve asssy. Ask Defender to sell you a Zodiac valve tool to have it handy.
With a medium Phillips screwdriver check that small pin screw if having one is well adjusted fully in. When assembling back valve onto tube with valve tool, apply soappy water on lower valve and fabric and screw back where originaly seated, visible marks needs to match again. Valve must be adjusted tight against tube's fabric for a tight seal..
Happy Boating