I hope i'm understanding you right Mark.....
I've done plenty of these soffits with the slits on the underside. I WFP them to get them clean, then you will get run off and drips. You can see the dirt collected in the drips. What i do then is put a plain rag (old T shirt) over the brush,maybe twist/tie it on and run over it. Its never failed to remove the dirt drips. I'm not sure what you mean when you say it was sticking to brick work as i don;t rub the rag along the brickwork and i'm not sure how you do? Also, once you have the underside wet try to concentrate on scrubbing with the water off rather than an inccesant flushing of the vents. The more you flush the more dirt will come out, you don't want to clean out the vents so concentrate on the outside of them.