This is either a bad design or brake shoes that swell when wet..
Which is rather hard to believe..
But, this is also another comeback on the dealership..
Some cars use a proportioning valve which limits the fluid pressure to the lightly loaded rear of the car...

Does this vehicle have anti-lock brakes??

Yes, it does have anti-lock brakes. When I first took it in, the dealer said, "Did anyone explain the anti-lock brake system to you?" Again, it was lucky that the mechanic experienced the problem, because he said (right after nearly beaning himself on the steering wheel), "Well, they're not supposed to do that." Thanks for your ideas; we're going try our local tire, brakes garage, and will bring your thoughts to them.

What is the type of your rear brakes: Shoe or disc?

What a Toyota that's not perfect??? VBG Sorry couldn't resist.

There isn't much your local tire/brakes store can do with this if the dealer didn't get it the first time. The most likely cause is the linings being too agressive when the surface of the linings have absorbed some moisture. Replacing them should reduce, or maybe even eliminate the symptom for a while. In many ways this is a "problem" that cannot be resolved at a technicians level, because we have no way to test different lining materials in order to find ones that wont cause this symptom, especially without creating a different one. Even if you do take this to the tire/brakes store, they should install new O.E. shoes on the rear, brake shoes from the dealer that is.

I'm not a car expert, but the owner's manual says they are disc brakes. A further complication is that they are regenerative; the braking action recharges the battery. The moisture absorbing lining concept is intriguing - although wouldn't those be standard for this type of car? I've never heard of this being a problem for other Prius owners (although I only personally know 2). We have another appointment on Monday - we will take any and all suggestions with us so as to be fore-armed with questions for the dealer. Thanks for all of your feedback!

This is a bit of a puzzle for disc brakes. Shoe brakes can exhibit moisture sensitivity, due to their built-in regenerative braking geometry. In very rare cases, the primary and secondary shoes can be interchanged and this will cause lock-up for sure. However, disc brakes are devoid of these problems. Disc brakes may be affected by brake materials and rust on the rotors. The rust will be mostly removed on the first application of the brakes. The other culprits are the porportioning valve and regenerative braking.

Regenative braking can be a suprise depending on the charge condition of the battery pack and what gear is engaged when the regenative braking kicks in. If the battery pack is not fully charged and the transmission is in one of the lower gears, the alternator/starter may send a sizeable shot of current to the battery pack and this will be felt by the driver. However, this affects the driving wheels only, and in the case of the Prius, I think it's a front driver.

So, why does your rear wheels lock up? Keep us posted. We'd like to know.

Regenerative breaking has nothing at all to do with the brakes themselves. That is actually accomplished through the "transmission", and the "generator" for lack of better terms.

As far as it having disc/drum, well first that just goes to show that we aren't seeing any of these in for any services at all. Virtually everything that needs done on these so far is being done at the dealerships that I know of. Brakes can grab, or be noisey, especially during the first one or two applications each day, and not be anything really wrong other that the moisture content of the surface of the lining. In order for them to even verify this problem, you really should drop the car off today, for Monday, so that it sits and recreates the same conditions you encounter. I know thats not always convienent, but it can really help in the verification of the issue. From there in order for the shop to test and verify a repair has been made, they actually need to allow the car to sit and try it again after a sufficient amount of time, which means a Tuesday pick-up at the earliest is in order IMO.

Hello - I have just seen your post regarding the prius brakes. I also have had this exact same problem. Will you please tell me if the reason for the grabbing was ever discovered and if there was a solution. Thank you very much for your time.