156 V6 doesn't have anything like this, the fuel pressure regulator is integral to the fuel pump, and its a 'returnless' system.Ah! It had a picture for me. Here is another link: SAAB ALFA FIAT GM OPEL RENAULT PEUGEOT 3.0bar FUEL PRESSURE REGULATOR 0280160560 | eBay
If your regulator has a rubber hose connecting the steel pipe to the inlet manifold then you should see more than 2psi variation as you open the throttle, but the engine must be on load to see it more than momentarily. Another check would be to (carefully) pull that hose off the regulator: do you get petrol from the regulator, indicating a split diaphragm?