The low side oring/seal on the AC compressor went bad so I replaced it. Im now trying to recharge the system but running into a vacuum issue. Vacuum doesnt seem to go beyond 27inHG on the low side gauge. The high side gauge doesnt appear to attempt to drop at all. As soon as I close the valves on manifold, my Vacuum starts to drop back to 0 on the low side. Here is what I have tried.

1. Replace the schrader core on both low and high service ports.

2. Test vacuum on the manifold - connect both service valves to storage ports and ran vacuum. No vac leaks and held for a few hours.

3. Closed the service valves while vacuum was connected and running - drops straight to 29-30inHg without vac leaks. I think its safe to assume that my manifold is good.

4. I can partially fill the system with r134a. No leaks at all. I can even charge it to the point where the compressor turns on.

So the system holds pressure but not vacuum. What else should I be doing and where else should I be looking?