i used to valet cars and the wheel acid we used was brilliant. it was really strong stuff (industrial strenth). i've used the stuff you can buy from car accesory shops and i don't reckon they're much cop. pop in to your local car valet garage and try and tap them up for a bit of wheel acid. don't leave it on for too long though. you should'nt have to scrub the wheels if it's good wheel acid, just spray on and leave for a few mins and then jet wash off. if the brake dust is really caked on, you may have to repeat the process a few times.
hope this helps mate, let us know how you get on.