If you go to the Evolution threads, you'll see everyone complaining about the "grey goo" that comes out of a new engine.

It's rather viscous. If it does not plug the fitting, it will collect in the pressure line as you've described.

You'll see less and less of it as time goes on.

By the time you have about 3-4 gallons through the engine in flight, you'll only need to periodically check for any gunk.