There are six o-rings between the solenoid and the manifold, and between the manifold and the marker body - three on either side of the manifold. Sounds like one or more of these could be leaking.
You mentioned needing the solenoid screws - is one of yours missing or broken? If so, that would explain the manifold o-rings leaking. If the screws aren't tight enough (this goes for the two screws holding the manifold to the body as well), the o-rings wouldn't be compressed enough, and would leak.
Make sure that the o-rings are in good condition, clean them, lightly grease them and then reassemble, ensuring that the manifold screws and the solenoid screws are reasonably tight.