I own a 2008 Chevy Cobalt-- Does anyone know if there is a way to fix the rollover valve in the gas tank without buying a new gas tank ?

 

how have you determined its faulty? Extremely rare for it to be a problem. If your issue is slow to fill tank...its more likely to be a blocked evap vent path.

 

Had it tested by a mechanic when they took the tank down to inspect for leaks, it will leak if tank is full and it stops leaking after driving it down to almost 3/4 tank. Do you think it's safe to travel 3000 miles like that ?

 

leaking fuel is not safe. you can't fix it. the valves are built into the tank. new/ used replacement tank required.