Central heating problem air sucked in through vent pipe

I have an open vented system in which I recently changed the pump, even on the pump lowest setting air is being sucked in through the vent pipe. The setup looks like this:
Boiler > coldfeed > vent pipe > pump > 3 way


There is about 1 metre between the cold feed tee and the vent tee (the pump is above cold tee), there was a block in the cold feed pipe which was replaced and drains freely ? Is this piping incorrect causing air to be sucked in ? Can this be fixed by moving the cold feed tee above the pump ?

Any advice greatly appreciated.

Thanks