I would think you would see the vacuum slowly rise, not hold steady for 30 minutes. There may be a problem with one of your o-ring installations, perhaps it seals during vacuum, but leaks when pressurized. Did you use a metal instrument like a screwdriver to get the old o-rings off? If so, you may have scratched the sealing surface enough to cause a leak. To see if the UV dye leaked from the evaporator, try getting some water to flow out the drain hose underneath the HVAC plenum, underneath the car. You might have to get to the blower motor, remove it, and spray some water at the evaporator and check for the UV dye when it exits the drain hose.