REPLACING AC COMPRESSOR?

I'll pull a vacuum just to see if it holds as a quick leak check. Then I'll pull it apart, flush the system out and replace the drier, tube and the EVAP core (if the compressor came apart) then swap the compressor. Then pull a vacuum and see if it holds. That lets me check for leaks and pulls any moisture out that got in while it was apart. For oil, the new compressor should come with oil, generally you drain the oil from the old one, measure it and add the same amount to the replacement. If however you had to flush the system I measure the compressor and then look at the total oil charge and what you drained out. Add the same amount you drained back into the compressor and the rest gets put into the rest of the system. GM went to a premeasured 8 ox cartridge on those vehicles, so say you drained 4 oz from the compressor, you would add 4 into it, then 2 into the EVAP core and 2 into the condenser core.

I started with a vacuum and after 30 minutes it was at -29. Left it fir 30 and still there

Should be OK to proceed then. Although this is a Hummer, it's actually a Tahoe 6.0 engine and 4L65E transmission. Simple replacement, pull any 134A charge out. Remove the AC belt and unplug the connector, remove the lines from the unit then the bolts that hold the compressor to the mount. With those out remove the compressor. Now dump out the oil, what you are looking for is debris or blackened oil, both mean the system needs a full flush and then the drier, o-tube and the EVAP core should all be replaced to keep any blockage from happening. Then you put it back together, adding oil to the compressor and other components, pull it down, check it, if it's good, charge it up and test it.

Replaced all including all o-rings, dryer. Clean oil came out of the dryer and compressor. Removed oil from compressor and added fresh oil half to dryer half to compressor, installed and it's cold, hopefully it holds and no leaks. Guess what, forgot to replace the orifice tube ugh

Good to hear. Clean oil is always a plus. OOPs... lol.