Friend, don't over-think this. If you performed gravity feed, air will not push through and out of the calipers. Air rises and gets trapped at the MC which would cause pump to cycle constantly. It takes about two minutes to crack the MC line fittings. I take it that you are in the snow belt? After the first MC line bleed, drive in the snow and trigger traction control and ABS events to get the valve block to cycle. Air can get trapped inside ABS valve block. Repeat MC line cracking. Anyways, there is some good news here. Pressure sensor appears to be working. And system does eventually build up pressure which also means accumulator could be good.