gas tank

[SIZE=2]Help – I had a leaky gas cap. So I put a new seal on it. The next time I got gas I could hardly get the cap off. A lot of vacuum. Upon gassing up, the tank would only hold a little more than 5 gallons. The tank had collapsed! So, how to fix the problem? I’m afraid to blow it up with air (in the car which would be the easy trick) because I may blow and rip the tank. I think, remove the tank and pressurize with water... Anyone had this problem before?
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I don't think that using air to open it back up is such a horribly bad thing but water sounds safer...

When in the holy berjeezus can a fuel pump collapse a fuel tank like that?

What you using to pump fuel, a briggs and Stratton Mud Hog?

Just the stock mechanical pump. Believe it or not. Turns out the vent got plugged.
Go figure?
Thanks for the reply.