Using an Ac Vacuum Pump, Having Issues

This is from my original post

" * First I got a junky parts store rental pump and followed instructions to check oil.

  1. Block off suction port, let run and check oil level
  2. Slowly add oil as necessary

As soon as I turned it on, freon started blowing out of the oil pour cap, so i tightened it down and tried again. Same result. So I figure it’s just a shot pump and bring it back. They swap it out for a new in box unit and I bring it back and the same exact thing happens (but with just air this time.) Am I just having bad luck or is this normal?"