Would like to make sure my thinking is correct. Injection pump delivers a metered amount of fuel to nozzles. Valve in nozzle detrmines injection pressure. Return fuel comes out of small orifice in injector body. Copper washer on seat of nozzle and oring on top of nozzle body seal fuel in return manifold directed to rear of head. My question then is why do we have return fuel. If these were unit injectors (electronic or mechanical) I could understand, but with mechanical nozzles how is there return fuel?