Random rear brake noise, and parking brake

1. One of the known endearing quirks of the Prius is that the friction brakes get so little normal use that they will pick up flash coatings of rust any time the conditions are right (usually, the car has sat for several hours or overnight in rain or drizzle or just really humid conditions), and every time that happens, the next time you drive the car it will make horrifying scraping sounds the first several times you brake. All you do is shift to neutral while moving (at, like, residential street speeds) and brake to a stop. Being in neutral inhibits the regen braking so the stop is made completely with friction. If you just do that once, maybe twice, every time you've started driving and hear the horrifying noise, you'll see it's all smooth and quiet again, just as easy as that. It becomes second nature, get in the car, drive off, hear the noise at your first stop, drive, neutral, stop, stop, done. Surprised you didn't see that on PriusChat already, it's been talked to death.

The really unfortunate stories are from the people who only post after they let the noise convince them something was really wrong, and they spent hundreds on a completely unnecessary brake job ... sometimes more than once.

Of course if you ever have weird brake noises and that procedure doesn't clear it right up, then you may really have something to post about (and be sure to say that, so we know).

2. The Prius rear drum brakes are of the type that self adjust through use of the parking brake, not through use of the normal brake. You don't have to be moving. Each time you apply and release the parking brake, the shoes will adjust by 0.03 mm if they need it. Obviously, if they've gone out of adjustment by many times 0.03 mm, you'll have to apply and release the park brake many times to get them set.

So, if the car is driven regularly by someone who doesn't routinely use the park brake, that's a likely explanation for the brakes getting out of adjustment in the first place.

However, if you have been using the park brake every time you park, and the adjustment has gotten loose anyway, and it doesn't improve with a bunch of uses of the park brake, it is possible the adjusters are gummed up. The mechanism is all right there for you to see, you just have to take the drums off.

If an adjuster gets gummed up and stuck in one spot for long enough, it can get one or two teeth of the star wheel sort of rounded off (because the adjust lever has just been slipping back and forth over it, instead of turning it). If that happens, it will probably always get stuck again when it comes back around to that position, so if the teeth are not all straight and sharp, you just replace the part. They're cheap.

-Chap

 

Kind of like that. Barrel with toothed wheel, yes. But in that picture it is down between the bottom ends of the shoes (where it usually goes in the design that adjusts by using the regular brake). In your Prius, you'll find it above the hub, below the hydraulic cylinder, interlinked with the parking brake lever.

It should be really unusual and unlikely for the brake cable to have stretched, unless one of you is gargantuan and always applies the brake like a strength test.

(In a past vehicle, I saw that they had designed a tension limiter into the parking cable: there was this steel rod bent around a post, and if you ever really overdid the pedal force, the rod would just unbend and pull around the post. The dealer could just replace that one part, and mention you don't have to stomp that hard.)

-Chap