Depends what caused the brown. Usually its a yellowing colour caused by chemical change of the polypropylene due to UV exposure. This generally can't be removed or renewed, however i've heard conflicting reports about using whitening toothpaste (huh yeah) or you could try a special plastic cleaner designed for cleaning plastic boat parts (you can get this at a chandlers).
I have a couple of machines I need to clean, so let me know how you get on, and I will likewise.
