T500 reflux overflow pipe

[jpsobral]

Dear all hope you are having a wonderful weekend.

Today for the second time, I am using my T500 reflux (I usually do a lot of potstill). I am having a problem that I really don't understand why is happening and force me to put a overflow pipe on the exit of the reflux condensor.

Let me explain the setup:
A water pump pushes water at a constant rate to a needle valve which is them connected to the top of the reflux condensor. At the exit of the condensor I have a temp probe and the exit tube of water, which goes back to the sink.


The problem:
At the exit tube water I notice I am getting back pressure - pressure forms and releases in air bubbles a bit of water. Then the system normalises and repeats it again. The water was going out where the prob hole is but now I put a back pressure tube (red tube in the picture). Nevertheless, every 30min or so the tube (red tube in the picture) will release air and water and the systems goes back to normal!! What's creating this back pressure?


Pictures attached.