12 gallon seat tank expansion

[Donnier]

I have an old polyethylene 12 gallon seat tank. I usually fill it to about 11 gallons and after is sits for a while, the top of the seat where you sit, expands enough to lower the fuel in the site line by about a 1/2 gallon or so. The weight of sitting on it compresses the seat back to its normal configuration. Is the flexing of the top surface of the seat an issue? How much flexing can it take?

Don

 

[gyrojake]

If it's the Jazz seat converted to a tank, it will be fine. Should only put 10 gals in it though.
How about a picture

 

[Donnier]

It is a Jazz seat tank but did not realize it had been converted to a tank. It had a couple of pin holes in it near the top back of the tank which I plugged and following the repair is when I noticed that the tank seat was swelling after filling it. After thinking about this, I wondered if maybe the tank was holding pressure. Sure enough, when I checked the gas cap, I found that is could inhale but not exhale. Removed the check valve from the cap and now is is vented both ways. No more pressure.