The engine on my '83 300D recently, and suddenly, became slow to start on the first start of the day. A bit of investigation seems to suggest that the fuel is flowing back to the tank as the vehicle sits. Given that there are no signs of external leaks, I am wondering if the problem isn't the suction/delivery valve in the fuel pump. Does it act as a check valve? I note that service manual recommends that the valve be either cleaned or replaced whenever the pump is removed. Does anyone have experience with this valve?