There's only a few things that could be contributing to the marker leaking down the barrel.
1. Cup seal. Sounds like you replaced it so unless you used a faulty cup seal, it's probably not that.
2. Valve face. This is the bottom of that plastic-y spaceship looking thing that comes in direct contact with the cup seal. It has to be smooth and level. Verify that it is so. If there are tiny burrs or bumps on it, then it might need sanding down to make flat and smooth again. I've tried highlighting the aforementioned areas in red in the pic below.
3. There's an o-ring that goes below the plastic-y spaceship looking thing that seals it against the bottom of the lower receiver. Make sure that o-ring is good. Also, make sure the walls of the hole where the spaceship-y lives are smooth and not marred by tool marks or other idiot marks.
Below is a pic to help you understand what I'm talking about.
4. Verify the spring is good.
Everything should fit in there like this.
And look like this