RE: 2004 Dodge Ram 2500 Cummins - fuel/water separator
unless you truly suspect having water in your tank then I wouldnt worry about draining the fuel filter, however there is a yellow **** at the bottom of the fuel filter canister that is on the drivers side of the engine. just flip that thing around and it will drain the fuel filter. just make sure you close it before you go to prime the engine.
if you were to change the filter yourself it takes a 1-1/8 socket or a large crecent wrench will also do the trick. pull the lid off and the filter comes with it, pull the filter off put the new one on and make sure you replace the o-ring and apply some clean engine oil or diesel to it for a good seal, before you put the filter back in make sure the canister is clean, a clean cloth can be used to wipe out the inside of the canister, then screw the cap back on but dont tighten it too much or you'll crack it since it is plastic.
to prime the engine back first make sure the drain valve is closed, then turn the key to run, but dont start, you will hear the lift pump running, after the pump quits running turn the key quickly to start then let go, dont let the engine start, all your wanting to do is make sure the canister is full of fuel. this is called bumping the engine. do this again and wait for the lift pump to turn off then go for a full start. if you did it right the engine will start right away.
personaly I try to check for water every 5K miles since I mainly get my fuel from high volume supliers, less of a chance of crud and water. you would be amazed what collects at the bottom of a fuel stations diesel tank[:'(] I have never had water in my tank.