I have a problem on an Mercedes Benz Actros MP1 3348. The fuel which is delivered to the fuel filter housing returns to the fuel tank. If you prime the fuel the housing of the fuel filter fills with fuel when you stop priming the system the fuel returns to the tank. What could be the problem, im thinking that it probably is a relief valve which is not working. So fuel goes into the fuel filter housing via the fuel in line (from fuel tank) and exits via the overflow pipe (back to fuel line).