In my experience, the problem is not the valve seal, although that may be why the cylinder warning sticker tells you that it is illegal to transport them if they have been refilled. But in order to get a decent fill on the little tank you have to chill it in the freezer while simultaneously warming the big one up to get a decent pressure differential. Even then you don't get a full 'fill'. It's quite a bit of messing about for not much reward. I gave up on that after doing it a few times and started looking around. I found the appropriate hoses and adapters are available all over the place, usually for about $10. Now the only green cylinder I have is the one my son uses for his Airsoft pistol and he does the refilling on that. I do not miss them at all.
