Gas pressure build up

Are you sure it is the fuel pressure and not a dead cylinder? How many pound of pressure does it develop? You might try adding a return line or a pressure regulator.

are you running the mechanical fuel pump? these pumps will not build pressure in the tank. they are a suction pump and will cause a vaccum in the tank if the vent lines are clogged. if you have a mechanical pump, and are getting fuel in the crankcase it's either one of two things. first is the fuel pump diaphram is broke. or the needle/seat valve in the carb is stuck. if you have a electric fuel pump, were is it mounted? they need to be as close to the tank as possible. they also will not build pressure in the tank, and they usually are not strong enough to pull much vaccum hench the reason to mount close to tank. if you have electric you need to measure your fuel pressure at the carb. anything over 5psi will cause problems. if it's more then you need a regulator. if you are running the mechanical pump you need to check pressure at carb also. some will put out over 7psi! if it's over 5psi then you can either put a regulator on it, or stack gaskets under the pump to lower the pressure. if you really do have a pressurized tank you got serious problems. maybe fuel lines hooked up backwards on pump?? take the gas cap off and run the car. do you feel air blowing out of the gas filler? do you feel it sucking air?

differnt person, same typw of car and same problem. my car actually built up enough pressure in the tank that the tank inflated like a balloon and when i took off the gas cap it spewed out like a volcano. car would not run with all this pressure built up but runs fine with gas cap loosened. the temp outside is a blitering 100 degrees if that matters