O-ring #5 in the diagram must be a -211, not a -210. This O-ring serves double duty...it is in a wide groove and must slide up and down to work properly. A -210 is too tight to function reliably but a -211 is just big enough to allow it to slide to each side of the groove on the up and down strokes.At some point along the way, Sun Optics posted some updated instructions on their site and it has this O-ring identified properly.http://sunopticsusa.com/content/HP%20REPAIR%20INSTRUCTIONS.pdf