on a lower entrance of housing 200 0031 fig2 shows the detailed configuration of piston 700 and its neighbor elements being in the resting mode the piston 700 comprises an inner part 710 being in contact with the lower portion 510 of hollow rod 500 and an outer part 720 being in contact with the inner wall of housing 200 in which the outer part 720 is rather compressed the outer part 720 is configured to extend laterally from the bottom of inner part 710 and then further downward than the inner part 710 in particular the outer part 720 is spread outward so that the lower diameter thereof is larger than the upper diameter accordingly the outer portion 772 of the outer part 720 comes into contact with the inner wall of housing 200 in the rather compressed state the piston 700 is engaged to the lower portion 510 of hollow rod 500 so that in order to secure the engagement a projection 712 is formed on an inner surface 710 of piston 700 meanwhile the stem 800 comprises an axial part 810 being inserted into the bottom of hollow rod 500 a horizontal part 820 extending horizontally from the.