Hi,
I am in the exact same boat as you. I have now had 2 H115i RGB Platinum's both with the same issue. Sadly I think its a design flaw with the coolit pump. I spoke to corsair customer service and they suggested all the usual - shaking the rad, moving the case while the pump is running, running with case on side over night qith pump on extreme etc etc.
I have been unable to release the air bubbles from the pump.
I am sadly at the stage where I am looking at going with a different brand or getting a noctua. Between me and my father we have had 6 corsair AIO's and all have been fantastic. Would be a shame to have to move away from them.