The gas cap has a one way vent valve which is to relieve VACUUM. This is to prevent tank collapse if there is a vacuum in the tank. Relieving PRESSURE at the cap would completely negate the entire purpose of the EVAP control system.
Positive pressure is relieved through the purge valve in the charcoal canister. If the purge valve opens, the pressure is then blocked by the TVV (Thermo Vacuum Valve), which opens when the engine temp reaches about 130F. If the purge valve and TVV are both open, then the pressure is blocked by the purge port in the throttle body which opens when the throttle is opened past a certain point. If all of these valves are open, then a blockage in any of the hoses between the charcoal canister and TVV, or the TVV and the throttle body, could be the problem. There is a very remote chance that the throttle body port valve could be plugged.
It might be possible to work back from the throttle body, removing one hose at a time, to locate the source of blockage.