For reference I'm working on a 1986 Corvette at about 5,000 feet above sea level (I'm in Colorado). The outside temperature is about 80 degrees, and humidity is relatively low.
The old R12 system had supposedly sprung a leak and the system had emptied, so I decided to convert it over to R-134. I flushed the evaporator, condenser, and all of the hoses separately, and installed a new compressor and orifice tube that came with the conversion kit I ordered. I also replaced a couple of the O-rings that were warped on the hoses. I now have everything hooked back up with the new R-134 valves on and a new accumulator in place, and am in the process of pulling a vacuum on the system before I recharge it.
The vacuum only gets down to about -21 in/Hg, and won't go any further even though I let it run for at least 2 hours. The thing is, when I close the valves on the gauge the vacuum will hold in the system at that point when I let it sit for an hour and a half, which leads me to believe there aren't any major leaks in the system.
What are some possible reasons the vacuum wouldn't pull close to -30 in/HG like it should, and is absolutely necessary the vacuum gets this low to remove all the moisture?