To orient you, and someone please verify, the fitting at the opposite end from the vacuum fitting (actually it's a vent to atmosphere) is the return to tank line. The small side fitting is the return from fuel distributor line. The big side fitting is the fuel pressure line.
It seems to me that fuel pressure is a signal to the regulator. The regulator regulates injector pressure by modulating the flow of excess fuel from the fuel distributor to the fuel tank. It does not look like excess fuel should go from the pressure line to the tank through the regulator as it does with PWM systems.
If you're saying that what you blow into the big side fitting is felt at the small side fitting or end fitting, that sounds like a leaking regulator. The chamber fed by the big fitting should be isolated in all normal circumstances from what I understand in Charles Probst's book on Bosch FI systems. Again, I defer certainty to Steve and comrades.
If you want to try the regulator in my 91 SE, I can be without it this weekend. Contact me off the list. Please tell me it's easy to remove and reinstall
As a check in the absence of a fuel pressure tester, crack a fuel line at any injector while cranking the engine. Is there fuel in the line?