Last night I went for a short flight, on take off gear came up good. Upon landing when I put the gear handle down I got nothing. I could hear the pump running but no 3 green. It just so happens I had my gopro under the wing so I could see that the gear was only coming down about 6 inches. I had to engage the emergency extension for it to go down and lock. I tried raising the gear again and it did fine. Did the same thing again when I tried to lower it. My AP thinks there is air in the system or it's low on fluid. What could cause this? How do I get air out of the system?