N14 starting problems cranks FOREVER !!!!!

I have a truck my dad drives its a 99 volvo with a N14 some time it will fire right up and other times it will crank for up to 45 sec and the batterys start getting weak before it starts cummins told me thats just a N14 but i find that hard to believe and why does it only do it sometimes here is what i have done so far

1.put on all new fuel line and return line
2.tryed the tube type fuel filter that cummins told me to use
3.it has around 50 psi of fuel pressure
4.had the over head ran just cause it was due
5.pulled the fuel line off the steel line where it goes into the head and its getting fuel there as soon as it starts to crank
it doesnt smoke when it starts and it run great other wise oh and when you pull the fuel filter off there is only about 2 inch of fuel in the bottom of it thanks for any help i can get

 

celect plus

 

can these be tested or just replace them

 

got ya, ,ok well i will try this and see what happens
thanks

 

new check valve and even bought another after market and put in