My 97 3500 sat up for about 4 days.When I returned and tried to crank it,I had difficulty starting.I had to bleed the fuel lines to get started,I suspect a slow fuel leak and was wondering what you think is the best method to detect this. I am thinking about dye and blacklight as the leak is'nt obvious.Would like your input,thanks.

 

I used 5 psi air in the fuel tank to find all but 1 leak and it turned out to be the filter drain it leaked under vacuum

 

I always use the air method also.
So much so that I have an old fuel tank cap with an air fitting tapped into it.
Also works excellent to fill and bleed the system after a filter change, no more pumping the primer button 200 times.

 

If you have a 97 diesel it most likely is fuel return line that it located left rear of engine. A real $#@% to get to. I would suggest removing two bolts that hold the steel lines return and supply together on the transmission. The loosen both hose clamps from engine. Pull fuel lines down and remove with steel lines still attached. Then you can replace both hoses at same time. 5/16 hose longer hose and 3/8 supply fuel hose. Good Luck