gastank presher buildup causing problems

hay everyone. i need advice again. im the kid with the 1975 dart 4dr special edition with the 225. i need to know if the gascap is suposed to vent the presher out because mine isnt. it caused the diaphram in the fuel pump to blow out. i pulled my breather off yesterday to find gass all over the intake and steadly bubbling out of the carb. it filled the cylinders and now i have to do another oil change after only driving 5 miles on the last one. i need help again.