Sounds like your technique could use a little work, but once you've got the first stubborn lot off, and sealed the wheels, it should no longer be a problem....
Personally if they were really quite dirty I would have:
Sponged off with warm water and fairy liquid (kills grease quite well)
Rinsed with pressure washer or similar
Sprayed on bilberry, left for a bit, agitated well with brush, in every nook and cranny
Rinsed with pressure washer or similar
If they're still bad, go over again with bilberry
Sponge off again with fairy liquid
If they come up nice and clean, seal them, well, this'll make the job 10x easier in future.