I think you're saying if you pump it to 10psi it leaks off right away but if you pump it to 12psi this doesn't happen. Assuming you have thoroughly wetted the metering needle using a light oil so it doesn't stick or hang then I think this is a problem.
That makes me think MAYBE the o-ring around the brass metering orifice is lose and leaking still, doesn't fit the bore or groove properly. By applying pressure you may be pushing the o-ring into position to make a seal (going under the assumption the o-ring is moving around).
I would suggest flooding the area around the metering seat/orifice with liquid and observing if there's a leak around the o-ring area, if yes then you may need to inspect the o-ring very carefully(using a magnifier) and make sure there are no nicks or cuts in the o-ring, also try to make sure the o-ring isn't rolled over with a twist in it at the injection molding marks. The o-ring should fill the brass groove completely and extend beyond the diameter so when the seat is placed into the bore the o-ring seals well between the two surfaces.
If fuel leaks past the o-ring(or the metering needle), the carb won't meter properly and you will experience excess fuel (flooding), most likely during shutdown the slight fuel pressure from the native vapor pressure in the fuel tank will force fuel up through your carb and cause heat soak flooding, resulting in difficult warm/cold restarts.
So yeah, try to find where the pressure is leaking from if possible, you don't want fuel leaking past your metering assembly.
BTW, it's the vacuum in the carburetor bore which comes through the small passages in the bore and into the fuel chamber, this vacuum pulls the rubber metering diaphragm inward towards the metering lever and pushes the metering lever thus lifting the needle off the seat to allow fuel to enter the fuel chamber. If there is no vacuum, then the metering needle/orifice is closed and fuel cannot enter the fuel chamber unless the fuel pressure exceeds pop-off. ie: pop-off changes dynamically with engine vacuum, your pop-off pressure check establishes the base calibration for the fuel metering.
