Engine/Transmission (1994 - 1998) Hard start/ Tank pressure issue

[jclark5810]

I was helping my cousin today with his truck (it's a 96') that's been having some hard start problems. We did the usual fuel filter change, check for leaks, check the solenoid, etc. I noticed while I was crawling around underneath looking for leaks that his fuel tank looks like a crunched up water bottle. Like it's been under so much pressure that it's sucked-in big time. What could cause that? What venting do these trucks have, or are supposed to have?