Troubleshooting vacuum pump

[GuzZzt]

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?

 

[duncsuss]

I have a T-junction on my pump with a gauge right there and a valve (to which I connect the hose). It lets me measure the vacuum the pump is pulling without anything else connected.

 

[Gary Beasley]

The seal on the tank should be thoroughly checked, expect too such a large tank may take a while to stabilize at max vacuum. Also you might want to check the accuracy of your vacuum gauge, it may be off.