I had that problem when I first installed my catback. All we did was unbolt the muffler from the h-pipe, twisted it, then tightened the bolt up and has been fine since.
I too had the same problem with my mac system. I was always adjusting it and then noticed it began to bend the brake line. It wasn't a big deal to adjust but I just got tired of doing itso I went with the bassani side exit system. It sounds and looks great. No problems at all........