Brakes not making contact

I noticed something crapy the other day...It was raining earlier in the day and ya know how the rotors look a little rusty after a rain? Well after about 5 miles of stop and go without acess braking I smelled that horrible burning brake pad smell coming from the left rear rotor. When I parked, I checked it out and left rear rotor was nice and shiny but the right rear was all rusty looking. What could cause that pad to not come in contact with the rotor. After messing around a little bit, I found that it does touch the rotor, but only if I really nail the brakes. I'm getting it looked at Tuesday, but you know how mechanics are. I'm not sure if I'll get a straight answer or not.

water can get between the rotor and the pad. when u brake, the car veers really badly (happened to me in my old bmw --2 disc brakes).

bad caliper/piston/seal, air in the line, water (condensation) in line. have you ever bled or replaced your fluid? have you checked to see if there is a leak?

whatever they tell you isnt working propertly, be sure you get the "damaged" parts back.

Thanks a lot guys. That's a good thing to know about getting old parts back but it was something completely different. According to the "experts" the caliper was badly rusted and wasn't working because of that. They took it apart and cleaned everything and they got it work. I guess I'll be buying new rear calipers soon though.