First rule of Palmer's: Don't take it apart.
Second rule of Palmer's: Don't take it apart.
You're probably Ok since it's a SS though
Could be the cup seal, or the o-ring that goes in the middle section of the valve. I suppose it could also be your main spring is too strong and is pushing the hammer against the valve pin slightly, opening it a smidge and letting it leak.
Really I'd just put it back together and try oiling the hell out of the valve. Make sure you have it oriented with the hole going up, as well. Palmer's brass love them some warm water baths and don't mind some oil either.