All you need is a Y-piece or T-piece in the pipe from the pump to the hose reel. One branch of the Y-piece goes to the hose reel, the other goes back to the tank. The valve (a simple on/off ball valve - any plumbers merchant sells them) goes in the pipe that goes back to the tank.
So: if the valve is SHUT all the water goes straight to the hose reel/pole.
If the valve is partly OPEN then some of the water goes back to the tank - use this if your varistream isn't in circuit so the pump can't build up too much pressure. Also use it to control how much water you get at the brush.
WARNING: If you HAVEN'T got the varistream in circuit, don't run the pump with the valve completely shut because if you disconnect your pole, the pump will just continue to build up pressure until something goes BOOM