The Injection Pump Overflow Valve is located on the side of the injection pump. It is also used to connect the fuel return line (banjo fitting) to the fuel injection pump.
It consists of a spring and steel ball, as the pictures below:
Fuel volume from the fuel transfer (lift) pump will always provide more fuel than the fuel injection pump requires. The overflow valve (a check valve) is used to route excess fuel through the fuel return line and back to the fuel tank.
Clean area around overflow valve and fuel return line at injection pump before removal. Remove valve from pump and banjo fitting and discard old sealing gaskets. Blowing against the valve, If it’s ventilated, proved the valve is bad, because if it doesn’t work, more Fuel volume will be transferred to fuel tank, then the injection pump can’t work. Then we need to change a new one.