I think I found what I needed. According to a service manual:
The fuel tank can be filled to approximately 90% of its capacity. The additional 10% of volume allows for expansion of the fuel without escape to the atmosphere.
When the engine is switched off, the fuel tank pressure is maintained at a positive pressure of 1.0 to 1.33 psi by the pressure control valve (aka Rochester valve). Pressure above 1.33 psi is released by the valve to the charcoal canister.
When the engine is running, manifold vacuum acts on the pressure control valve, which opens the vent line from the fuel tank to the charcoal canister. Air enters the charcoal canister and flows to the tank to replace the fuel delivered to the engine and maintain atmospheric pressure in the tank.
If the pressure control valve fails, the fuel tank cap will vent the fuel tank pressure at 2.0 to 2.5 psi.
When the canister purge is enabled, the ECM meters purge flow to the intake manifold through the normally closed evaporative emission control (purge) valve (EVAPP). Canister purge is enabled by the ECM based on engine coolant temperature only when closed loop fuel metering control is operational.
Purge Flow Monitoring for OBDII
The ECM detects purge flow in two ways: if closed loop fuel metering correction indicates a large movement toward lean when purging is enabled, or if the idle control valve corrects for increased air flow when purging is enabled, the ECM has confirmation that purging is taking place.
Evaporative Emission Control Valve (EVAPP)
The new, redesigned evaporative emission control (purge) valve is a normally closed pulse width modulated valve. The amount of valve opening (and canister purge flow) is determined by the ECM drive signal allowing the ECM to accurately control purge flow for the prevailing engine operating conditions.
What I think is happening, is that the Rochester valve I have installed is not releasing pressure below the threshold of the fuel tank cap, so the tank is being exhausted into my trunk, hence the fuel smell. The whoosh I hear is the 2.0-2.5psi thats still trapped in the tank.
When I pulled the Rochester valve, I turned it around because it was indeed much easier to blow out of the CAN side than it was from the TANK side. I reattached the vacuum hose. The problem could be that if the vacuum fails, I may starve the fuel tank of replacement air.