An injection assembly (1) of a die casting machine comprises valve means for controlling the advancement and return of the injection piston (20) and of the multiplier piston (42). The valve means comprise a plurality of valves, including a proportional delivery valve (104), a proportional injection discharge valve (112) and a proportional multiplier valve (118). Electronic control means (300) are provided which are configured and/or programmed to execute an air purge cycle which performs the opening and closing of the valves, alternately, for a predetermined number of times.