DD13 fuel filter change/hard to start

Do you fill filters before putting them back on

 

Try shutting the tank valves off before taking filters off so fuel doesn't flow back into tanks and drain fuel lines

 

Best guess.....Primer pump seal is allowing air to enter.

An old trick I hesitate to mention,
Remove diesel tank filler cap, using compressed air and blow gun wrap it with a clean rag to form a seal and pressure up the fuel tank. Obviously you need to use some common sense here as a couple of PSI is all you need to push fuel up the system. I hesitate to mention this as common sense is not some common today and I can see someone blowing out a face full of fuel by over pressuring and / or removing the sealing rag to quickly.
This was a common method in the days of the screaming Jimmy Detroit 671 engines if you ran one dry on fuel.

 

Even easier trick locate tank fuel vent hose apply a small amount of pressure to each tank and the check valve in the vent keeps it pressurized .. pump primer a bit and it should fire right up... do not forget to slowly open each tank cap to release pressure.. I've started reefers and my old cascadia this way after having to change filters on the road due to gelling

 

That's what I used to do with my old 8V92 anytime I had to prime the system. I used to take out the breather which is a 1/4" BSP fitting and screw in a fitting to take an air hose. Then remove the return line and plug it's entry into the fuel tank. Pressurise the tank at about 2 psi until fuel is pouring out the return line.

 

On the filter housing is a test fitting. I use a 2 gall bug sprayer and connect to the test fitting and pump and wait till sprayer is almost empty then it will fire up. Otherwise priming pump Is suppose to be pumped around 200 times then wait and another 200 times or similar to start.
Bug sprayer is cheap and I keep it full of diesel to wash parts and such with. Saves alot of time.

 

I usually have to push that primer button about two hundred times three try's. I recently got myself a compucheck fitting so I can make the bug sprayer primer ClineER mentioned.