My truck's odometer just rolled to 50K and in need of a brake job. I noticed the left front rotor is in really bad shape (deep groves but only about half of the outer part of the disc) compared to the right front rotor. I'm not sure why this is happening. Anybody encountered this before? What would cause this to occur? How can I prevent this from happening again? Any input or advice is greatly appreciated.
Thank you.

BYW, when I hit the brake, I can hear the left front brake making this hissing, clamping noise (not squeaking). I'm not sure if this means anything.

 

make sure and rebuild your calibers when you do new pads and rotor,your two bolts that the caliber slide on probably dried out(no lube)and or piston bore is shot,you probably have no lining left on your pads and your grinding metal to metal when you apply brakes