In all fuel pre-pumps of DB two of these return valves are mounted. During one valve opens, the other valve closes and the movement of the piston is converted into a pumping movement. Using the fuel pre-pump, the fuel injection pump is filled; it will reach 0.3-1 bar.
If the return valves do not work reliable, you will receive performance degradations due to inadequate filling of the pump elements.
material: brass, diameter: 18 mm