I just saw that the front disk brakes are not engaging at all on 1999 Toyota RAV4

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I am changing the front pads but half worn and see rust on the disk as if it has not engaged at all.
(1) Answer
Drive it down the road and hit the brakes a few times and look again. If both pads are worn the same amount then brakes are working!
Also it would be pulling to one side if it were not engaging right and have a poor pedal. Common for rotor to have surface rust after sitting overnight due to moisture in the air!