Need help troubleshooting

[C.Meyer]

New to the forum.  I recently purchased a used 2015 sierra 2500HD with the 6.0l gas engine.  It has 95k miles on it.  I've noticed that it cranks much longer after its been sitting for more than an hr or longer.  It usually starts, but sometimes doesnt.  It always starts right up on the second crank if it doesn't on the first.  I've noticed that if I turn the ignition on and wait before starting, it seems to not have a problem.  It also has no problem when in use, if it just sits a short period of time it fires right up.  

 

It seems like a fuel pressure problem to me but not sure.

[JD110]

The check valve in the fuel pump assy. is weak. Replace pump and sender in tank.

[2017HD]

Could also be a clogged fuel filter.

[mjm-1957]

Does not have an external fuel filter.Put a fuel pressure gauge on the fitting on the rail and monitor it for pressure drop when shut off.The fact that it starts right up after turning the key on and waiting to crank sounds like What JD110 said above about a leaking check valve in the fuel pump module. The waiting pressurizes the rail and it starts normally.