qr against a seat on the wall 24 on the outlet side on the inlet side the spring qr is compressed between the wall 24 and an end stop provided at the end of a valve stem although the valve q has been depicted only in the alternative form of fig9 it is obvious that it could also be provided in the embodiments according to the other figures that being the case the way in which the motor and the metering device work is as follows let us consider a starting position corresponding to the one illustrated in fig2 the piston 9 is at the end of its downstroke and the rocker h which has just changed configuration has raised the valve cage 31 and the plungers 23 a 23 s the inlet ta for pressurized liquid is connected to the lower chamber c 1 while the outlet ts is connected to the chamber c 2 the liquid pressure is exerted on the underside of the piston 9 across its entire cross section and causes this piston to rise the liquid in the chamber c 2 is delivered to the outlet the piston plunger 15 ascends in the tubular element 16 and can draw an additive from a container connected to the coupling 18 at the end of the upstroke the piston plunger 15 raises the link rod 32 and causes additional compression of the leaf spring 41 when the pin 42 crosses the position of alignment with.