Low speed brake "grind"

yes, the computers apply both regen and brake pressure in most situations. the harder you press the pedal, the more friction braking and less regen. down to around 7 mph, then no regen.

this transition point throws many people off, and can even feel like the car is shooting forward if you're on rough terrain, even though it is still slowing down.
prius brakes are typically very grabby at low speed, and rust can build up on the rotors increasing the problem.
putting it in neutral once in awhile arond 30mph and gentle braking will clean the rust off.

 

Does the "slip" light come on the dash (ie, ABS is operating?) If not then it sure sounds like something is up with the pads.

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Just a thought that may not be related...

Have you check the slide pins, they maybe stuck..

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Don't lubricate the brake caliper slide pins with regular, petroleum based grease though. It will degrade the rubber seals and dust boots.

Use a lithium soap or silicone base grease that won't break down or swell the rubber seals and dust boots.

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At 7 MPH the regen stops and transitions to 100% hydraulic braking.

If you are getting a rumbly noise and/or shudder when less than 7 MPH I wouldn't worry too much. Just consider it an alarm to alert pedestrians who have lost all situational awareness.

 

More likely you've just got some transient build-up related to the unequal pressure from sticking slide pins.

Drive more. Do a few hard brake applications to help scrub down any inappropriate build-up and let the pads develop a new face on the rotor. Do not expect instantaneous results, but keep track of it. Now that the pieces are back to articulating the way they should, it will take a few miles for everything to find a new equilibrium.

 

Suggested to bed pads and rotors after installation. This is the procedure for my coated rotors. May be different for your brand.

If you’re forced to a stop during deceleration, either shift into neutral or give room in front so you can allow the vehicle to roll slightly while waiting for the light. The rotors will be very hot and holding down the brake pedal will allow the pad to create an imprint on the rotor. This is where a judder can originate from.

How to: Break-in Your New Evolution Coated Rotors - PowerStop Brakes

 

If you stopped from a high speed in 'N' then that could be the case. In a Prius, the rotors will likely be relatively cool.

 

I would be slow to assume the actuator for an issue like that. You would essentially be assuming that it is applying fluid pressure on that side when you aren't braking. It should be pretty easy to do sanity checks on that assumption, say by cracking the bleeder open on that side (while you aren't braking) to look for any sign of the unwanted pressure you'd be assuming was there.

I was in a similar situation once, and even bought a couple electronic pressure transducers, and adapter fittings to mount them at the bleed-screw holes, so I could chart any pressures there while driving (without braking) and catch them in (what I thought was) the act.

That would work, if there's ever a situation where it's really needed. But in my situation, I was overcomplicating things. I had a caliper without the proper piston return.

I also tend to pay more attention to things like that, and to the "fitting kits" (those springy, Teflon-coated clips that hold the ears of the pads) than to the slide pins. I just did my usual every-tire-rotation brake inspection, and just as I have always found, all eight slide pins were just fine, no reason even to remove them. The rear fitting kits, though, needed replacement again, and for the second time since I've owned the car.

My recommendation is to just look at the whole system of caliper, mount, clips, pistons, and slides, and check everything for doing what it's supposed to, and attend to whatever isn't. If you find a pins problem, fix it, but in my experience that's not nearly as often as you'd think from reading PriusChat, where you'd almost think they're the only parts of the brakes.

 

Ah, dash lights! Do you have the codes yet?

When I had the caliper with zero piston return, the piston could be pushed easily enough in and out by hand. It just lacked that expected 0.3 mm or so of natural return that its rubber seal normally gives it when the pressure is let off.

That was when I made my resolution to always check new or rebuilt calipers for that before mounting them on the car.

 

The repair manual does not call for any lubrication of the points where the pad sits in the front disc brake pad support plate (which sits in the caliper bracket) as these have a Teflon coating that should provide a slick enough surface on which the brake pads can slide. If this is what you meant by 'anti-rattle hardware' then that should be cleaned off both the pad ears and the pad support plate.

I used to grease the support plates as I thought I read in the manual that some special Toyota grease (Brake Caliper Grease p/n 08887-80609) should be applied, but I think that was in relation to a special service campaign to address squeaky brakes in a different model of Toyota and is not applicable to Gen 2 Prius.

There should be a small amount of brake shim grease (not the Lithium soap base glycol grease used for the slide pins, nor the above-mentioned brake caliper grease) used on the face of the anti-squeal shim that faces the back of the pad. Normally this is supplied in a 5 g sachet (p/n 08887-80409) as part of the anti-squeal shim kit and is freakishly expensive to buy on its own, (like US$25 per sachet). Most will use copper anti-seize for this purpose if they run out of the supplied grease.

 

The slick coating eventually wears off those clips, at which point, if you're not replacing them, a little grease might be better than nothing. Or maybe not. Toyota would just have you replace the clips with new slick ones. (I think they frown on grease there because it's so exposed, and would just collect grit.)

I just did some rear brake work including shims, so I got that little sachet of shim grease. It comes with a SDS that identifies it as Molykote AS-880N, if that means anything to anybody.

"Good water resistance" is an understatement. Just try to wash that stuff off anything. Guess that's why it stays on the shims.