pct no . pct / us79 / 00859 sec . 371 date oct . 15 , 1979 sec . 102 date oct . 15 , 1979 pct filed oct . 15 , 1979 pct pub . no . wo81 / 01034 pct pub . date apr . 16 , 1981 in prior art hydraulic systems , and in particular in hydraulic systems having a cylinder means for elevating and lowering a load , there exists the possibility that the hydraulic line from a control valve to the cylinder means could rupture , allowing the load to drop before the operator could react . the present invention solves this problem by providing a blocker valve in the vent line of a check valve which check valve is interposed in the main line 27 between a cylinder means and a control valve . the blocker valve is responsive to the back pressure in the main line from the check valve to the control valve . thus , when the pressure in said main line drops below a predetermined level due , for example , to a break in said line the blocker valve automatically closes the vent line , preventing the check valve from releasing hydraulic fluid , and thus preventing the load from dropping . in an aspect of the invention , a resolver valve actuates the blocker valve . the resolver valve communicates with the main line at two positions and selects the lowest pressure between said positions .