fuel tank sucking in on itself

i was out on my 1991 sj last weekend, and noticed that it seemed to be starving for fuel, so I took it back to the shore and pulled the hood, and noticed the tank was or had imploded as if it wasn't vented, so I pulled the vent line off the tank and it went back to normal, I noticed there is a check valve in line, what do you guys think would cause this?
 
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