Check valve:

Check Valve

The only solution is to prevent air coming back into the tubing that direct the ink into the waste ink tank but how?

I have tested a couple of possible solutions and one of these works very well. I bought a check valve ($2.50) which I placed at the end of the tube just before the ink goes into the waste ink bottle and this prevention works very well. It prevents the air enter the tube as well as ink flowing back into the pump. The downside however could be that this check valve is also a filter, filtering the waste ink is really not so necessary and may give an additional maintenance in cleaning it from time to time.

Last but not least I came up with the idea to place a man made check valve in between the pump and the outlet of the waste ink tube. This solution works very well, may last longer or even without any maintenance, can easily made by yourself in a minute using the tubing which is already in place (Click on the picture to enlarge).