shower air lock?

I've just installed a salamander RSP50 using a new dedicated 22mm feed from cold tank and 22mm feed from Essex flange in side of hot cylinder. there are pegler non restrictive quarter turn valves on input and outputs. The output goes back up to loft and I've put manual vents at the highest point. I flushed input pipes before connecting the pump and the flow is excellent. When I tried flushing the outputs the hot eventually came through but I cannot get the cold to flow. I'm guessing an air lock but I've tried blowing back through pipes and I would expect the vents to clear it anyway. Any guesses?

 

I had another go and sucked on it (without the vac ) and got water flowing, not as much as I expected but I'll let it run a bit. As the pipes are open I thought the water would just push the air out. Thanks for the reply.