Pressure normally builds up inside the gas tank with all cars. Back in the days fuel tank vapors (gases) would escape into the air. Modern cars route gas tank vapors back to the engine. Modern gas tanks are sealed.
If your car is letting out air from the gas tank filler whenever you unscrew the gas cap, gas tank vapors are not being routed back the the engine properly. Your car's EVAP system needs to be inspected. This fault is not related to the manifold crack you've mentioned. It is a completely separate problem having to do with the EVAP system. Either there is a plugged or pinched vapor hose or faulty EVAP purge solenoid.
