Sure. I'm guessing there's still some pesky air bubbles somewhere, making it tough for the pump to build the pressure needed. Looks like you got it functional, though so maybe you got all the big ones!
From a design perspective I imagine that pump is made to move relatively little volume, but at a very high pressure. It won't take much air for that design intention to fall apart. A little air will take a LOT more volume pumping to pressurize it compared to the nearly incompressible hydraulic fluid.