02lancerr

I recently had to replace my gas tank on my 02 lancer es i picked one up up at the junk yard due to the fact that a new one from mitsu was $530 and the junkyard one was $150 I hooked everything up put my old pump in and went to start it it took a little bit but it started. I figured it was natural due to he fact i drained lines. Well whenever i leave the car over night or more than a couple hours it struggles to start. Although once it starts it's fine doesn't stall or lose power and will restart no prob.

Any thoughts on what could be causing the rough starts? I wasn't sure if you maybe had to bleed the lines or possibly a vacuum leak?

Thanks