Was the brush and pole second hand?
ie. Has it been used before?
If it has, do as Tosh says, check first to see that your backpack is pumping water out with a good flow just through a bit of hose. If that is still a problem you know it's something to do with the backpack. If the flow is fine, then you know you have a blockage somewhere in your pole/brush. I had a blockage a while back..took all the backpack apart! Ended up being the little Y piece on the brush, where your pole hose goes to the brush, pull it all apart and clean through. You'll find it. It probably isn't the backpack if that is charged and running good.
Don't panic Gary, you sound like me, when something goes wrong don't think the worst, think of the simple, obvious things first!