I've also found that the glue holding the valve in its rubber sleeve can rot after a year or so - so there'll come a time when you try to remove the dust caps and end up pulling out the valves. A solution is to grease the dust cap threads before ftting, and to keep the pressure low to avoid a "blowout" (5 psi works for me). I found that rubber solution glue does not set, but it offers enough resistance to stop the valve popping out at a low pressure.

Of course, you can replace the valves (or tubes) periodically, but this solution will work when disaster strikes at the start of a sailing day.