gas leaking from cap?

[johnymac]

I realize now not to park the truck on uneven ground after filling the tank, but want to ask if this is normal.

When ever I fill my gas tank in my 67 C10, not to the top, but close.
When I park on a sloped graded, low on the filler side, it leaks. It leaves a big puddle, not to mention the gas makes a mess down the corner of the cab.

I have tighted the cap and learned not to park this way.
I am curious if this is normal, or do I have some kind of pressure problem?

Thanks in advance to all that reply!!