Brake Hoses, Lines/piping, And Fittings problems of the 2001 Nissan Sentra

I have had a problem when appling the brakes, with the car going into a slide /skid and almost hitting another car, going at a slow rate. This happened more than once. I was told on a couple of occ. That it would cost me to have them look at the brakes, even though the car was under warrenty. They said they drove it and could not find anything wrong with it. The last time I took it in, they found the car to be unsafe to drive and said it related to the calibers on the brakes. What can I do for a complaint to Nissan for their lack of responsibility on the situation? can I take legal action against them? dt.