Replace Dinghy Air Valve
In  the spring of 2011 I re-inflated my AB RIB for the first time in nearly a year.  The dinghy had been stored on deck since I deflated it in the previous year.  During the inflation of the bow chamber the little red, plastic stem inside the valve broke off.  The stem and the spring around it control the valve - closing it during operation and inflation and opening it during deflation.  I was still able to inflate the chamber and the pressure of the air in the chamber kept the valve closed after I removed the pump hose.  I tested it for leaks around the valve and found none.  I left the dinghy inflated on deck for two months with no sign of a leak. 
I did not use the dinghy until the fall of 2011, when I took a mooring in Annapolis, MD while working for the U.S. Boat Shows (separate sail and power boat shows).  Now I needed  the dinghy to get from the mooring to the dinghy dock in Annapolis harbor, and now the bow chamber was leaking badly.  It would almost completely deflate over night.  The valve which had previously stayed closed when the chamber was inflated now was stuck open and only the valve cap proved a poor seal for the valve.  I really needed to repair or replace the valve.

The defective valve, missing the red stem and spring		 On the left is picture of the defective valve, missing the red stem and spring.  At this point the valve had once more closed from the internal air pressure and the chamber was holding pressure or leaking very slowly.   I talked with the AB dealer in Annapolis and was told the valves could not be repaired, but could easily be replaced.  All I needed was a replacement valve and the tool used to install/de-install the valve.  The tool is not absolutely necessary, but makes the process much easier, especially for someone who has never done it before.  I purchased the tool and three valves from the dealer.
The valve unit consists of two pieces - the valve itself and the base into which the valve is threaded.  The base resides inside the air  chamber.
On the right is one of the valve units after I unscrewed the valve from the base.  When I replace the valve on the dinghy I will use the base that is already inside the chamber and just screw the new valve into that base.  There are no sealants or adhesives used with these valves.
Also in the picture is the tool used to install and de-install the valves.  This is a very simple tool, just a metal tube with two teeth at one end that engage the valve and can be used to turn it to screw it into or out of the base.
One air valve unit (separated) and the install tool

The new valve installed in the dinghy		 The first step was to remove the old valve from  the base.  To start I left the chamber inflated.  I inserted a screw driver in the hole in the tool to provide additional torque and turned the valve enough to start the unthreading from the base. 
Then I deflated the chamber.  This was necessary to to allow me to  grab the base from outside the chamber and hold it in place.  Replacing the base would have likely required the use of a heat gun to soften the dinghy material so the base could be pulled out of the chamber and the new one inserted. 
Since there did not appear to be any problem with the base, I just replaced the valve.  Holding the base in place, inside the chamber I screwed the new valve into the base, then tightened it with the tool and screw driver.
Then I inflated the bow chamber and tested it for a leak.
Bow chamber inflated.

Defective air valve.		 On the left is the removed air valve, missing the stem and spring.
This is the valve seat that seals the valve when the spring on the missing stem forces it up agains the bottom of the valve housing.  Air pressure within the chamber can also press this seat against the housing and provide a partial seal of the valve.
The missing stem piece fits over the stem on the seat and somehow is fastened to this seat.  That fastening is what is broken in this valve.
This appears to be an infrequent, but not uncommon problem with these valves.  I would advise anyone with an AB or other inflatable that uses these valves to keep a spare valve onboard and the tool in the tool box.
The Valve Seat