Every single time it rains, our 2005 Prius’ brakes tend to go haywire-and just when we need them the most. They back breaks lock up, bringing the car to a jolt and throwing us forward. Sometimes it feels like it won’t break and then bam, a very hard stop. Sometimes they squeak and grind too- It never happens when it is sunny outside-only where there is a significant amount of moisture in the air. We’ve taken the car to the dealer three times and they can never find anything wrong with it. Next time I bring it in, I want to have something with me to say- have you look at…or can your replace this…
It sounds like the rear brake shoes are worn to a point where they lock up the rear wheels when it rains. This can happen with any vehicle with rear drum brakes.
The usual fix is to replace the brake shoes and check the brake drums if they require replacement.
How many miles on the car and when were the rear brakes last replaced?
Because this car uses regenerative braking, the front disc and the rear drum brakes are not called into use until the end of braking and under extreme demand. In rainy conditions the discs can stay wet and build up rust causing the squeek and uneven braking. The rear drums may not be used at all on a regular stop so do not get dried out and possible rust scrubbed off. This would probably cause the rear brakes to grab.
One of problems with the brake system on the Prius is that the PCM takes over management of the brakes during the stopping, traction control, stability control, and ABS situations so a service technician would have to know the software strategy designed into the system. Even if the PCM is out of the braking loop, only the front discs will brake (one for each of the dual brake system). Under normal situations, when you step on the brake pedal, the PCM gives you a pedal feel and reads the brake pedal position to give the required stopping rate. During this time the front caliper and rear drum pressures are software driven by commands to the apply and release solenoids used for traction control, stability control, and ABS. There is a separate brake fluid pump to develop the needed pressure.
I know this does not answer your original question. You might ask a Toyota engineering representative if there is a way you can force the PCM to apply, dry, and clean up the brakes at each corner after a passage through a water level that would douse the brakes i.e. crossing flowing water or a deep gutter flow. If you do find a technique, please pass it along.
Hope this helps
Did you ever get this issue resolved? My 2006 Prius is doing the same thing.