after a lot of mucking around, i finally managed to make my wfp car washing brush, it works brilliantly, my backpack has a 40psi pump, but anyone with 60 or 100psi backpack will have even more pressure,
just use the on/off switch supplied with washing brush,
also buy the longarm brush adaptor, you need this or it wont work
if you have a big car, just leave a full 20ltr bottle on top off the backpack, saves refilling
heres a short video, showing it working, sorry not very good quality