I would check the gauge. If it really had 12psi going to the carb when you started it up cold, fuel would be pooring out the bowl vents.

I have seen liquid filled gauges do funny things when they are mounted in the engine compartment and are subjected to the heat. Barry Grant has a notice in the box when you buy one of their carbs stating "not" to mount liquid filled gauges near the engine or close to a heat source.

I have a Barry Grant 280 fuel pump on my car with #8 line to the regulator and #6 lines to the carb. No return line and the fuel pressure stays steady at 6 3/4 pounds. I will however have a return on the fuel side of the nitrous whenever I get around to installing it on the car.

It's just a guess, but it's worth a try.