midway of its length and radially extending bores 35 at the open right end the radially extending bores 33 34 and 35 communicate respectively with annular slots 36 37 and 38 located on the exterior of sleeve 31 the open end of bore 31 is closed by a threaded plug 39 a reciprocable piston 40 is located within sleeve 32 and a blind axial bore 42 extends from the open end 41 of the piston to the closed end radial bores 43 connect axial bore 42 with an annular slot 44 on the exterior of piston 40 at the closed end the outside diameter of piston 40 is slightly enlarged near the open end and the inside diameter of sleeve 32 is slightly enlarged at the right end the enlarged diameter of sleeve 32 extends into the radial bores 34 this results in an annular lip 45 at the outside diameter of piston 40 the annular slot 36 is in communication with a channel that is a part of line 10 or line 13 as then in fig1 of the drawings and receives feed pressure from the line the annular slot 37 is in communication with a channel that is a part of line 8 or line 11 and the annular slot 38 is connected to a drainage channel 46 that leads to changeover valve 14 radial bores 47 are formed adjacent to the open end of piston 40 and communicate with axial bore 42 the closed end 40a of piston 40 is constantly connected with a feed pressure from one of lines 10 or 13.