the fixed displacement hydraulic motor 56 is mechanically coupled to the additive pump 58 with a shaft 54 still referring to fig1 the fixed displacement motor 56 receives hydraulic fluid from hydraulic fluid conduit 46 this supply source will be discussed in detail below the hydraulic fluid from motor 56 is returned to the hydraulic fluid source 50 through conduit 48 that passes through charging pressure relief valve 96 and filter 68 before reaching tank 50 a separate return line 44 is provided for returning hydraulic fluid that accumulates in the case of motor 56 to hydraulic oil tank 50 this fluid also passes through filter 68 an additive pump control apparatus 42 such as a programmable controller or closed loop digital controller is provided for varying the rate of the piston motor 56 which in turn varies the rate of the additive pump 58 in the preferred embodiment the additive pump control apparatus programmable digital controller 42 is a eh129a1a control card manufactured by ditco inc of kent wash a power source 41 such as the power source from the rescue or fire fighting vehicle provides power to the controller 42 a switch or switching means 43 is also provided to turn the controller 42 u201c on u201d and u201c.