I went through the same thing with Cody. He didn't even let us touch him at first, let alone brush him. I would corner him in the kitchen, hold his collar with one hand and gently brush him with the other. If he growled or snarled at me, I would tell him no, and keep on with what I was doing. In the beginning, he got his mouth over my hand a few times, but never left anything worse than tooth imprints, no blood. I wouldn't put a tight collar on him, or restrain him, since he has the scarring on his neck from where the chain collar grew in, prior to him being rescued. He still wears a loose buckle collar. We got him in February and they blow their coat in late spring, so I knew he had to be used to it, in short order, so I could get that undercoat off him without it matting up horribly. It took about 3 weeks of me brushing him for an hour or so every night, before he stopped reacting badly. The main thing is to stay calm, so he will calm down, don't raise your voice, and above all, don't stop, when he growls or reacts. If you do, he won and will continue the behaviour. Good luck with them and you are doing a wonderful thing by taking in rescues.
Kathy