I've been hearing this grinding noise coming from the front left wheel when I brake. So I checked it today and the brake pad is all worned out and gone. Now why is the right side isn't affected. My left side is pretty much has no brake pad left and the right side is still pretty thick. What can I do to correct this besides replacing the right side pad? BTW I hydroplane the left side of my car into a guard rail and curb about a year ago.

 

Proportiong valve I believe is the one that distrbutes left to right. Check that.

 

Could also be a hung caliper. Does your front left rim seem to have a lot more brake dust than the other side? Try jacking up that corner of the car and spinning the rim/tire. If it doesnt spin as freely as the other, your caliper may be stuck. Time for a rebuilt one in that case.