[Beemer]

We verified the vent wasn't kinked or clogged. My neighbor has an earlier Mark III and we switched the gas caps and the problem went away. An investigation of the caps showed his inside was different than the one on my car. I spoke with Olthoff people and they said it is a common problem with the later model gas caps. They are too tight. The newer caps have raised rings under the gasket causing the problem. They had me remove the gasket and cut 3 equal distant spokes about a 1/32" deep from the center to the edge of the inside edge of the cap across the rings under the gasket. I replaced the gasket and it worked and the pressure in the tank is gone. Thanks all who commented.