Yesterday the brakes on my car started making the unmistakable sound of severe wear. As one might guess, this involves the front wheel brakes–disc brakes.
What is strange is that this wear seems to be only on the left front wheel rotor, visible because of the design of the wheel covers.
This happened before, on another car, years ago–on the RIGHT front wheel. Why do the brakes wear in this manner? It’s the same mystery as shoe soles that wear a hole on one shoe but not the other.
In my experience it’s usually because the caliper is sticking on one side and not the other. Disassembling the caliper and scraping off any rust if necessary, then greasing up the pins so that the calipers slide back and forth easily will usually fix it.
Thanx 
What e_g_c said … maybe bleed the brake lines … air bubbles in one side could cause this
Finding no better choice, I took the car to a local service place of a national vehicle service company (after having my charge account reinstated) and got repairs made. The brake pads and rotor on the left front wheel were worn down to the metal, they said (from what I could see, I believe it); on the right front wheel, despite the appearance, the rotor was “too thin to turn.” (I know what that means.) This set me back about $300, which I expected.