The nickel plated brass valves are prone to seizing if they're not greased. First thing to do is to dry them off, then put a rag into the valve. Spray Duck Oil Penetrating Release or Plusgas on the valve and clean off any overspray from the tubes. Do this and leave for a few days, and reapply if necessary.
Check out this earlier thread. Servicing Zodiac I/C Valves
I've attached a PDF for the 'o' ring, diaphragm, gasket, etc, to order from local Zodiac dealer. Key thing on reassembly is to use a decent marine yellow grease on the metal to metal valve components. Service periodically and you shouldn't have problems in the future. Good luck.