My son's 2008 has had a "ghost" brake grind noise in the rear that sounds like a metal on metal at low speed braking (15 mph or less) during or right after it rains. 2 dealerships, a general mechanic, and a hybrid/prius specific mechanic shop have all said nothings wrong and they can not duplicate the noise (mainly because it's dry when they are checking it I suspect) This morning my wife's 2006 started making the same noise at same speeds in a parking lot. Wifes car had state inspection less than 2 weeks ago and were told all brake pads have plenty of life left (brakes done 2 years ago by dealership) Anyone else have this happen/ got an explanation as to whats going on?
The rotors rust super fast in wet weather and then they either cause the noise you describe and/or some jerking in the 5 to 0 mph part of the stop. Not sure if the drums do the same thing. To fix it just get the car up to 30 or so, put it in neutral, and apply the brakes pretty hard. That will grind off the rust and they should operate fine for the rest of the trip. If one or two hard stops like that doesn't fix it then you have some other issue.
I find the being in neutral is the key. (In D, the car will mostly use regen instead of the friction brakes, which is why they don't quickly rub the rust off. Neutral makes them be used, so the rust gets rubbed off.) If I get in the car and start driving after a rain and I hear the telltale rust noise, I'll just quietly shift into neutral on my normal approach to the next two or three normal stop signs. I don't drive extra fast or brake extra hard or anything like that. If I have a passenger and they don't see me shift to neutral and they don't notice a thing different about my driving, I'm doing it right.
I'd say make more use of the rear brakes so they don't get old and noisy, which they are & can't easily be avoided once any car gets old enough. As in consider driving in reverse more often which adjusts the rear brakes and also use the emergency brake every time you turn off the car, which also makes use of the rear brakes and causes less wear&tear on the undersized "park" part that's a hassle to replace if it fails.
The rear drum brakes in a Prius do not depend on driving in reverse for adjustment. Toyota has used drum brake adjusters of two different types in different vehicles and years. In the Toyota training literature (available under the Library tab on techinfo), the kind you're thinking of are called "reverse-travel adjusters", and the kind in the Prius are called "parking-brake adjusters". The mechanisms look similar, except for the location of the strut and adjusting screw. In the reverse-travel type, the strut and screw are low, acting on the bottom ends of the shoes. In the parking-brake type used on the Prius, they are high, acting on the tops of the shoes, just below the hydraulic cylinder. You can easily pop the drum off and see which type the Prius has. You can also easily watch how it works ... as it doesn't require the car to be moving, and only depends on alternate motion of the parking strut and the hydraulic cylinder, it's pretty easy to see it do its stuff.
I have experienced the same thing on both our 2008's. Will try that in the future. I also recently did a brake job and found that my brake caliper pins were dry. I relubed them and since then the rubbing has been much much less. Just an FYI
