boom or the like all of which may be mounted on a tractor and be operated by the usual hydraulic apparatus employed in that type of installation and that will be readily understood by one skilled in the art in the aforementioned arrangement it will be also seen and understood by one skilled in the art that the apparatus 12 39 is reliant upon a hydraulic pressure in the line 18 39 in order to maintain the piston 14 39 and thus the work load 23 39 in the upwardly supported position of course if there is a reduction in the hydraulic pressure in the line 17 39 then the work load 23 39 will immediately fall and it is this contingency and danger that the present invention is arranged to avoid to do this the present invention provides the system shown in fig1 and it incorporates the interlock or safety valve 16 39 which is arranged to immediately respond to the loss of pressure in the hydraulic line 17 39 and the valve 16 39 acts to lock and retain the hydraulic pressure in the line 18 39 to avoid having the load 23 39 fall the safety or lock valve 16 39 has a housing 24 39 which has fluid ports 26 39 and 27 39 to which the lines 17 39 and 18 39 respectively are connected therefore it will be seen and understood that hydraulic fluid can flow from the valve 11 39.