I would say, open the mechbox again, clean everything (use some brake cleaner stuff. The one I have is Wurt and does not attack rubbers or plastics). Then check for debris, deformations of casting errors/bumps.
All I can see is that the mechbox shell was casted wrong, and either the rails are not aligned with the cylinder or there is a bump somewhere along it that cause too much friction.
Also look on the piston rails to see if there is a mark on the rails, or a spot with no grease at all. That can be the sign that the mechbox rails are binding.
Check for deformation in the cylinder/piston head too It might be a simple bend caused by the machines when they did it, and since the mechbox probably cycled once or twice, it passed QC.
Good luck, and if you don't find what is it, throw it at me instead of the window.
