The friction from the Lexus disc brake pads is what stops your vehicle. For the longest possible life, the pads must wear evenly and the surface of the rotors has to be as flat and smooth as possible. The best way to ensued the flatness of the rotor's surface is to replace the disc brake pads before they wear out totally and the backing plate they are attached too comes in contract with the rotors on your Lexus. There are two ways this time can be known. The wheels can be taken off the vehicle and the disc brake pads visually inspected. When there is a 1/4" or less of the pads remaining, it is time for a brake job on your Lexus. This is probably the easiest way of knowing. There is, however, another way to know if the Lexus disc brake pads are worn and in need of replacing. As the disc brake pads wear out, the piston in the brake caliper has to move out compensating for the loss of this braking material on the pads. This lowers the level of brake fluid in the brake master cylinder. The front reservoir in the master cylinder is for the rear brakes and the rear reservoir is for the front brakes. It is the front brake reservoir that should be used for this test. The reasoning behind that is because of the brake bias on your Lexus is set to be twice as powerful for the front brakes as it is for the rear. Because of this, the front disc brake pads will wear our twice as fast as those on the rear. If the owner feels they should be adding fluid to this rear reservoir, then it is close to the time the disc brake pads will have to be replaced.