I'm working on a 1972 Mercedes Benz 280 SEL with 4.5L and the D Jetronic fuel injection system. The car has been parked for 6 years and I have gone through the fuel system to clean and replace hoses. The car runs, but the fuel pressure drops to zero immediately after turning the engine off. I can cycle the key with the return line pressure pinched off and hold pressure, so that tells me that the check valve in the pump is working. Is the check valve for the fuel return to the tank in the fuel pressure regulator or the fuel dampener next to it? The car is slow to start because it won't hold pressure. While running, the car has about 33 PSI fuel pressure. Hope someone can fill me in. Thank you !! Jim