hi all my water is reading 0 but gettin a lot of complaints as there are spots/dirt marks on windows after ( not by me by my workers). can any1 shed any light on this? my water gets pumped from a tank to a tank in my van, gunna test its purity after it comes out of pole as im concerned its pickin up dirt while being pumped. my lads are experienced so dont understand why we are gettin the marks. if membranes or filters need changing then the purity would show this with a higher reading so ive cancelled that out. finally could not cleaning brush head be the victim as we have never done this. if so whats the best way to clean a brush head?