I've cut open one ford purge valve, and found that the rubbery seal which is moulded to the piston was literally crumbling. I could squeeze it with my fingers and it would crumble into bits. If I still have my other bad purge valve kicking around, I might try to cut it open too. I know, sample size of 1, for a problem that thousands have.
Doesn't make sense to me, it should be some material which doesn't dissolve in gas, if it's used in evap. Maybe it's something else like heat that causes it to go brittle too quickly. These are only warrantied for 36k mi by the fed emissions warranty thingy, so not like it has to last very long.