I'm in the process of correcting a fuel pressure leak down problem on my 84 500SEL, and thought I'd found the answer upon removing what I believed to be the check valve at the fuel pump. The piece I removed from output side of pump is wide open from one end to the other. After purchasing a new check valve, whose pump end threads are different, (it's for a Bosch pump) I started wondering if I need one attached to the outside of the pump, or if Pierburg has an internal check valve. The picture has the original piece on the right, the the non fitting replacment on the left. Hope some can shed some light on this for me. Thanks...Bob

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