The fuel system does Not hold pressure when the engine is off. Glow plugs have nothing to do after engine is running, only to warm up the cylinders to help starting. The return lines are 5/16" ID, if original. Any antisieze is good to use on the injector threads, does not seal, only keeps them from rusting in for future removal. Black smoke is too much fuel, wether it be from bad injectors, or pump turned up too much when driving. White smoke is lack of fuel. Smoking is normal at start up, maybe for a minute or two.