[johnm]

how hard are you sapposed to press against the window with the brush

[cybersye]

until it breaks then you realise you've pushed too hard 
seriously though you dont need to push too hard , just enough for bristles to spread slightly as you move up and down a window, for heavy deposits of dirt or bird doo I rinse, return to it with water off for a scrub then scrub with water on and rinse.