I have had that problem on occasion, but have found it usually clears itself, though I have had to on one or two occasions do the same as yourself.
When I am adjusting the height of the pole, or swapping from one pole tro the next I will either just fold and pinch the hose near the brush head, or if I need to stop the water flow for a little longer than a couple of seconds, I will yet again fold and pinch the flow off, and to hold the 'pinch' together I tie a quick knot in the hose.
Works a treat, the water pressure is maintained as the pump is of course running, undo the know when I am ready and off I go
If my explanation is a little unclear I'll post a couple of pics........if I can still remember how to do them that is
Ian