An optical transmission module includes a printed circuit board including a first surface and an opposite second surface, a ceramic substrate mounted on the first surface, a light emitting diode and a photodiode carried on the ceramic substrate, an optical printed circuit board mounted on the second surface, a first reflective unit and, and a second reflective unit. The guide grooves and the lands meet conditions expressed by:\u03bb18\u2266DG\u2266\u03bb11\u2003\u20031. Movement of the plug is retarded by a frictional engagement of the plug with an associated passageconduit. Accordingly, the valve assembly of hermetic compressor can make the valve smaller and simpler, so that reduced fabricating cost and improved productivity can be obtained.