I've owned the vehicle for 6 months.  Almost 1 year ago, previous owner had a shop replace front left wheel bearing as well as left and right calipers, pads and rotors.  A couple of months ago, I replaced the front right wheel bearing.

 

Suddenly yesterday, we started getting a grinding sound from the front drivers side.  It mainly happens when braking and it isn't constant.  It also happens occasionally when not applying the brakes.  When it does happen, it feels as if the brakes are being applied lightly.

 

I've pulled off the wheel and removed the caliper assembly and rotor.  I did not find any debris or damage to the rotor.  Nothing unusual with the brake pads, plenty of life remaining, with uniform thickness end-to-end and side-to-side.

 

I did notice that one of the two pistons in the caliper was extended approximately .25" more than the other.  Also, on one of the plunger rods, the little protective boot was not attached at the outer end, and the grease inside is pretty dry and dirty.  I'll clean them up and put new grease on them.

 

I would appreciate your advice.

Did you check the wheel bearing for play?

 

The pistons will move independently of each other and one can be extended further than the other when the caliper is loose, but both should be in contact with the pad during normal operation.

 

Put the caliper back on, or put the pads in the caliper with a block of wood roughly the same thickness as the brake rotor between them, pump the brake pedal and check to see that both pistons extend and make contact with the pad. If only one is working the caliper needs to be replaced.

Yes, I did check for bearing play and there was none.

 

I did as you suggested with a wood block. Both pistons did extend and make contact with the pad.

 

Could insufficient lubrication in the plungers be the cause of what is happening?

Possibly. The slide pins can stick and jam the outer pad against the rotor if the caliper can not slide properly.

 

You might also make sure the backing plate isn't dragging on the rotor. 

Cleaning and lubricating the slide pins seems to have solved the problem.

 

Thanks for your help!