Eating rear brakes every 5000 miles

What's the condition of the fitting kit (the little clips where the pad ears slide, 04948 here)?

Those come from the factory with a dry, Teflon-like coating (no grease, would only attract grit). It wears off.

The channels they fit into in the caliper bracket can sometimes rust; as that rust expands, it can squeeze those clips down on the pad ears.



Have you measured the caliper piston return? More on doing that in this post.

Key to a good measurement is minimal pressure; on the bench I used barely enough air pressure to get the piston to kiss the test block and then return.

There have been threads with attempts to measure piston return on the car by stepping on the brake, but at the kinds of pressure the brake system can exert, elasticity of the metal parts will figure into the measurement and throw it off.

 

how many miles on your car? i'm calculating over a million at 10k/month

 

Hmmm...that is a mystery! One thing the rear brakes have that the front doesn't is the parking brake...check that to make sure it really releases fully after use.
One other thing is how hard are you on the brakes? With hybrids, we're told to "glide" as much as possible and to brake gently and early, not "slam" on our brakes as this both increases battery charging by using the regenerative brakes and also only uses the hydraulic brakes a little. Slamming on the brakes will always use the hydraulics. I know a lady who is a "gas and brake" person and she's only getting MPGs in the 40's in her Prius.

 

I think the v has a parking brake design different from the Gen 3 liftback.

The liftback has parking integrated with the rear disc brake calipers.

I think the v has a top-hat-shaped rotor, and the disc brake pads are only used as the service brakes. The parking brake uses a separate pair of brake shoes inside the top-hat of the rotor.

 

The Prius v has a mini-drum brake for parking brake, so it’s not the “X” orientation of the piston face bugbear.

 

the regen system is in the tranny, but brake pressure is controlled by ecu. you might need tech stream to try and see what's happening, idk. surprising you're not getting a code.

 

Shouldn't be doing that. Have you tried 'B' mode down that hill?

 

Connecting a scan tool and datalogging the pressure sensor readings from the brake actuator would be a good way to confirm or disconfirm what you think is happening here. (The Tactrix Openport 2.0 is a J2534 dongle with a handy feature, a memory card slot where you can have it do its own datalogging without a computer connected).

That kind of datalogging relies on getting the pressures sensed by the brake ECU and returned in CAN messages every so often. Another option would be to get one or two pressure transducers and plumb them into the brake lines and log those in real time, using something that actually logs their signal voltages. I bought a couple such transducers (from Dwyer) years ago when I was troubleshooting that Gen 1 brake issue, though it turned out to be simply the zero piston return of the aftermarket caliper.

There is an outfit ssbc-usa.com that sells a lot of oddball brake bits, including sets of threaded adapters you can use to get from the standard threaded fitting on a pressure transducer to the caliper threads where the bleed screw goes in.

 

I doubt this is happening unless the rears are dragging.

I "seriously" doubt reversed operation as described in the scenarios above. Even if the brake booster was demanding hydraulics routinely, the fronts would wear first. We know the master cylinder is flawed on these models, my 2012 v wagon (260k miles) lost the brake booster system early, but I have never heard of reversed actuation.

Dragging brakes, yes, but as a high mile driver, with resources such as Toyota mechanics, independent hybrid mechanics and forums, I have never heard of reversed hydraulics as a fault. Without trouble codes it would seem to be near impossible with the level of wheel speed monitoring employed.

In my case, the front pads, calipers and rotors are original; the rear parts are original except for the pads which were changed about 5k miles ago simply because one rear bearing was dragging. Of course I am in a no rust state where the idea of brake hose or caliper replacement on a ten year old car is unthinkable.

Dragging brakes should cause low mpg and is reasonably obvious to careful inspection.

I have to think the parking brake actuator mechanism or the calipers are suspect. Do you use the parking brake? Were the new calipers aftermarket? Any brake booster codes?

 

Maybe chock front wheels and jack up the back, try spinning the wheels. Then have assistant apply release brake, then parking brake, while you spin them some more. Might reveal something..