2 days ago, i changed the GHL silicone tube for GHL flex PVC.
Problems solved, no airpockets or bubbles. No checkvalves needed, just as a good dosing pump should do.

As thought the problem is that the siliconetube is not tight enough on the connection piece (suction side) of the dosing pump. When the pump starts it draws a little bit air inside.
I don't understand that GHL promote silicone tube as standard.