the non return valve element 30 during the expansion stroke that is when the piston 10 moves upwards in fig1 the non return valve element 30 is forced to abut against the outer valve seat 42 and the inner valve seat 46 which is the upper surface of the support shim 22 that is both the outer and inner valve seats 42 and 46 are closed with the non return valve element 30 as a result when the piston stroke is relatively slow the working oil in the upper chamber above the piston 10 is forced to flow through the holes of the non return valve element 30 the orifice passage 44 and the through holes 38 of the piston 10 into the chamber below the piston 10 in this case the hydraulic shock absorber generates the damping force the magnitude of which is proportional to the square of the piston stroke or speed due to the frictional resistances which the working oil encounters when it flows through the orifice passage 44 it follows therefore that thus produced damping force is dependent upon the dimensions of the orifice passage 44 which is defined between the leaf valve element 20 and the non return valve element 30 and more particularly the thickness of the support shim.