i just got a new bike for my birthday almost a week ago. ive tried it out a few times and it seems like there might be something wrong with the brakes. The front brakes seem to work okay(althought they are very squeaky but it seems like the back brakes aren't slowing me down much at all. I applyed the back brake a few times and seemed like it was working but on my old bike i could use the brakes and they would slow me down more. I can't tell if I just have to get used to the bike or if there is something truly wrong with it. What does it sound like? What should i do?