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Usually when I start driving with my hybrid car (2016 Toyota RAV4 hybrid), I release the parking mechanism, select reverse, and the car begins moving very gently. Sometimes I have had an issue, usually after car has been unused for few days and the weather has been rainy, that the car doesn't move. When I slightly press the accelerator pedal, I hear the sound of something becoming unstuck and the car moves. The press of the accelerator this has required is usually so minor that I have considered this a "feature" rather than a "bug".

However, today after a month of driving very gently obsessing over the fuel economy (meaning the braking is 99% of the time below the maximum limit for regenerative braking), and subsequently parking the car for few days in rainy weather unused, the car had the problem again. It didn't move. I gently pressed the accelerator, didn't move again. I pressed the accelerator more and more, and only when the accelerator was pressed halfway did it suddenly become unstuck and reversed suddenly, so suddenly that I had to immediately release the accelerator and immediately brake.

It took so much press of the accelerator pedal that I became concerned. After less than 10km driving very gently, during which I pondered whether the parking mechanism (the mechanism in the transmission; I didn't use the parking brake) or the brakes were the part that became stuck, when I parked the car again I took a look at the brakes. Front disc brakes were fine. However, both rear disc brakes were totally horrible. They were so covered in rust that I didn't see any shiny metal on the brake disc surfaces. I had never previously seen such badly rusted brake discs. Often previously I had seen minor surface rust in the brakes after being parked in the rain, but the next drive had removed the surface rust completely.

The next 15km I drove very aggressively, alternately accelerating to needlessly high speeds and braking so suddenly that most of the braking force had to come from disc brakes as it was way over the regenerative braking limit. During those 15km, the fuel consumption was 30% higher than ordinary for that distance due to heavy braking. Most of the rear brake disc surface is metal now, but there are quite major rusty grooves on the rear discs. They still don't look healthy to me.

My questions:

  • It it possible that a very gently driven car has rear disc brakes so badly rusted that the brake mechanism becomes so stuck that it requires half-pressing the accelerator pedal to release the stuck mechanism? Or is it more likely that the parking mechanism is the sticking part?
  • If a car has such bad rust on the rear discs that half of the accelerator pedal travel is needed to release it from the stuck state, does it sound like the discs are permanently damaged from rust or could very aggressive driving with lots of braking salvage them?
  • If the cause of sticking was the disc brakes, could drum brakes in the rear be less susceptible to sticking if I don't activate the parking brake on a rear drum brake car? (Of course the current car will forever have disc brakes but I'm certain my next car will be electric and I'm thinking whether it will be Skoda Enyaq with rear drums or Hyundai Ioniq 5 with rear discs.)

I'm somewhat surprised that non-actuated rear discs with pad clearance could become stuck from rust.

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The problem is two-fold:

  1. you left it stationary and the discs got surface rust.

  2. you brake so light that you hardly use the fronts and due to the brake proportioning valve (front - rear) the rears are hardly used at all.

So, you need to regularly drive this with sufficient braking and it may be easier to put some load in the back to force the brake proportioning valve to change the balance front to rear which will help keep the rear brakes functioning.

If the rear discs are badly scored then they may need replacing.

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  • The thing that is sticking is most likely the parking brake, which is a drum brake independent of the rear disk brakes. Cleaning up the parking brake drum (inside the rear hub) will probably fix it.
    – alephzero
    Aug 10 at 13:35
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    @alephzero do all rear disc brakes have internal parking brakes with shoes? Or do some use the "standard" rear pads and a mechanical clamp?
    – Solar Mike
    Aug 10 at 13:42
  • No idea Solar Mike, but I looked up a video on brake service on the RAV4 and it does. If you just take a rear wheel off to try and diagnose the problem the disk pads and drum are visible, but there is nothing obvious to show you there is also drum brake unless you know enough about cars to wonder how the handbrake works.
    – alephzero
    Aug 10 at 19:34
  • The sticking may be in the emergency cable and not be the brakes themselves. Aug 10 at 19:44
  • I didn't use the parking brake at all, not sure if it could be the sticking brake.
    – juhist
    Aug 12 at 8:55

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