Pump could have seized after 10 months of no use, did you try anything to free it off before changing it? I would suspect you have either a blockage or an air lock, or possibly both now. Pulling some water out from the middle of the system wont tell you much, again if the system hasn't been run for 10 months, all the crud will have settled out in the low spots.
I'd also try running a decent strength magnet along the pipework around the pump, and especially where the vent and cold feed enter the system, if there's any sign of it attracting then that is a sign of a blockage. (You will need a stick to clear this....)