the closing member 27a has an effective cross section a and together with the housing 25a forms a chamber 30a this chamber 30a is connected with the duct 17a and thereby with the hydraulic cylinder chamber 5b of the pump 1b with the pump supplying paint into the duct 8 to the spray gun 113 furthermore the housing 25a comprises a chamber 31a with a piston 32a the latter is connected with a piston rod 33a which projects in a sealing fashion towards the housing 25a into the chamber 30a and presses onto the front face of the slider 26a when the chamber 31a is under pressure the hydraulic oil is supplied to the chamber 31a through a duct 34a by the oil pressure in the chamber 30a a force f 1 equaling a 1 u00d7 p sub 5b acts on the slider 26a wherein p sub 5b is the hydraulic pressure of the cylinder chamber 5b in the opening 28a a force f 2 equaling a 2 u00d7 p sub 5a in direction opposite to f 1 acts on the slider wherein p sub 5a is the hydraulic pressure of the cylinder chamber 5a of the pump 1a accomplishing the pre compression stroke while the force f 2 increases from zero with increasing pre compression in the pump chamber 6a and therefore with correspondingly increasing hydraulic pressure p sub 5a the force f 1.