I've never cleaned brass in mine (I use a tumbler), but for gun parts I use 1 part Ballistol to 8 parts water. Works wonderfully.
I don't know if Simple Green would hurt anything, but in my opinion it stinks, so I don't use it.
'Course, there are those strange people that don't like the aroma of Ballistol.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.