Abstract: Since its start up,the 35 MN hydrostatic testing machine located in a certain company had been bothered with leakage of the pin hydraulic cylinder.After investigation,it is identified that the failure is caused by damage of the cylinder's "O" seal ring.Accordingly,the original 2-position 4-way change valve of the pin cylinder is replaced by a 3-position 4-way valve with the "O" type median function.Thus thanks to the self-locking operation via the median function,the non-rod chamber of the pin cylinder is well protected from the impact as occurring when the hydrotester's stop device is loaded with the pipe.As a result the above mentioned leakage failure is permanently fixed.