Sprayers are finicky things sometimes. It sounds like you could have a small particle blocking the seal in the top ball. (the one inside the piston) Another hint although unlikely, I once repacked a Graco 5900, and even though I'd done it probably 10 times on various machines (we had 4 identical 5900's) I somehow managed to put the plastic washers on the piston backwards. BRILLIANT!
Don't know if that helps you or not, but good luck!