Yeek, yeek, yeek or sometimes pads don’t wear evenly and you’ll wear down the pad that doesn’t have the warning indicator. In that instance you may get metal to metal rough grinding sound. Don’t worry the car still stops but just sounds obnoxious during brake applications. This second variation is common in the rust belt states. Where high humidity, rain and Winter salt can rapidly cause rust formation on the rotors overnight and if you don’t drive the car daily, can turn the rotors into sanding blocks prematurely wearing away the pad material.
When should one change the brake pads ? When they are worn down to less than the recommended minimum thickness. BUT.....given the miles and the age, you are way, WAY past the recommended interval for other brake service....like fluid for instance. I suggest that NOW would be a good time for a complete brake job. Brakes are kind of important.
IIRC, 2mm is the minimum thickness allowed by Toyota, but the caliper pistons and slides could easily be troublesome well before that wear level is reached.
When should one change the brake pads? Not really. You might be able to sneak a peek at the outboard pad if the wheel spokes are positioned just right, but you need to inspect both to ensure they are wearing equally. You should service your brakes every two years or 20,000 mi whichever comes first. This should include removing the pads and cleaning of the pad "ears", the top and bottom mounting clips (Brake Pad Support Plates) and the caliper frame (Disc Brake Cylinder Mounting), reassembly and re-lubing the appropriate spots (as indicated in the repair manual). The removal and clean of the slide pins and regrease with the Lithium soap base glycol grease (or equivalent). Inspect slide pin boots and apply a little Lithium soap base glycol grease. While you have the brake pads out, measure the "meat" part of the pad and ensure it is greater than 1 mm. 1 mm or less, replace. Factory pads are ground on the short ends in such a way that when you can no longer see the ground part they are less than 1 mm. Note: If you have inspections, inspectors not familiar with the Prius can start giving warnings from 4 mm and even fails before 1 mm. So you need to know the pad thickness and know the Toyota recommended minimum (1 mm) so you can push back against an incorrect assessment. If you do this regular and routine maintenance, you will get long life with even brake pad wear.
