I bought a vacuum pump (2 Stage, 2.5CFM, 12/3 HP, 3x10-3 Pa) that I have connected to a 12l chamber. I get a vacuum around 27" Hg but not better than that. I expected it to get closer to 29". I can see some bubbles in the oil window on the pump, not sure how to get rid of that. I have tried twice to change the oil but it only slightly improve. How can I troubleshoot this?