Prius front brake pad replacement

(after 200K original miles!)

After reaching five or six years of Prius ownership one of the fun aspects was telling people how, nearing 200K on the clock, the car still had all its *original* brake pads and shoes with their linings still well within safe limits. The look on some of their faces was priceless, but many of them in the same breath while saying "What?!" realized that it was largely due to regenerative braking in the hybrid system. And my conservative and predictive approach to driving lent them that much more longevity before getting down to the point where I'd consider any replacement.

But wear they inevitably would, because no hybrid can stay *completely* off its binders all the time even though the second-gen Prius does a really stunning job at avoiding it under most circumstances. The front pads -- which take the vast majority of braking load -- were starting to get down there, and the Toyota guys even noticed and mentioned that when they were doing the wheel bearing work. I assured them that I knew exactly how far down the pads were and was planning on changing them relatively soon but not that day thanks just the same. I was originally thinking to wait until after 200,000 miles just for bragging rights, but with an impending long roadtrip coming up and likely not enough local driving to push me over 200K before then I went ahead with it around 196.

And the primary reason the pads were as worn as they were was exactly said roadtrips, when I'd go tooling around in the mountains with lots of elevation change. Under absolutely idea conditions the Prius battery can absorb the car's potential energy at about one vertical foot per available watt-hour of storage, or theoretically 600 feet of drop. In real life it's more like 400 or 500, after which regenerative capture is a distant memory and you're on brakes and B mode for the rest of the descent. Knowing all this, I would even occasionally bring the E-brake pedal into play a little on long downhills to try and balance a little more of the inevitable friction toward the rear. Didn't seem to make much difference, though, as at this point the rear shoes still had plenty of meat on them and weren't anywhere needing attention.

Yeah. *Drum* brakes with original shoes sailing handily through 200K and beyond, even with a bit of extra deliberate abuse here and there. But their life is pretty easy.

Pump and park relays pulled Anyway, it was the right time to do the fronts. A complete set of new pads was about $70 at the local dealership, and a while later came a nice not-deathly-humid day to attack the project. Safety first! I didn't want the pistons sailing out of the calipers from stray hydraulic pressure getting to them, so first thing was to depressurize the whole system. There's a scantool/Techstream/whatever command to do this, but it's easy enough to do without it. I pulled both "ABS MTR" relays, the blue ones lying in the cover, and worked the system by slowly pumping the full brake pedal range until the accumulator was empty. Halfway through the process the high-pitched low-pressure alarm beeper started screaming, which I simply ignored and kept pumping. Eventually it was pretty clear from the pedal feel that there was no more assist pressure to be had. After a bit of inactive time after I stopped pressing, the warning beeper went silent.
The relay I'm holding is the parking-pawl relay from "P CON MTR", which I also pulled so I could leave the whole drivetrain in neutral. With the car still powered up in IG-ON I put it in Neutral and then pulled the relay, thus disallowing going back into Park at power-down. More complaining from the car, of course -- "parking system is improper" and the like, but I knew what I was doing and wanted to be able to spin the front wheels.

Piston is pretty far out So it was kind of amusing to think that it had taken nine years for the caliper pistons to travel a half-inch. And I was about to undo all that. There was no evidence of leakage and the rubber boots looked in good shape, so I wasn't too worried about retracting back over dirt or the like. Kind of astounding that the piston seals withstand bursts of 2000 or more PSI and never seem to weep any fluid at all -- or if they had, it was still contained inside the boot someplace and in the interest of *not* introducing dirt unnecessarily I wasn't going to open them up.

Corroded/frozen bleed valve The original plan was to crack the bleeder valves just a little bit to ease the process of pushing the pistons back, losing a little fluid and replenishing at the reservoir later. However, the bleeder tit on this side had never had a rubber cover on it ever since I had the car, and whether or not that would have helped, steel threaded into an aluminum caliper had fallen victim to dissimilar-metal galvanic corrosion and simply wasn't budging even after a little splash of WD-40 and a bit of waiting. I didn't want to really wail on it at the risk of rounding the hex or shearing the plug completely off, at least not until exploring an alternative procedure.

Prying piston back into caliper Slow push back of piston
A look at the hydraulic diagram shows a couple of paths by which wheel-cylinder pressure might return to the reservoir. The most likely one for the left-front wheel I was on first seems to be via SLRFL, where fluid could push the poppet against the spring and squeeze past and not have to fight against valving in the master cylinder. In any case, simply pushing the piston back [which works on many other braking systems too] was worth a try. Gentle but insistent prying, first with the caliper partially reinstalled and then with a more direct approach as the gap widened, allowed the piston to s-l-o-w-l-y retract back into the caliper to start making way for the new pads. Yes, a hefty C-clamp would have been the correct tool here but I didn't have an appropriate size of one on hand.

The boots collapsed evenly right back to where they had started their journey almost a decade before, with no sign of anything bad happening inside them.

New vs. old brake pads The old pads were still clear of the wear-warning springs, but certainly near the end of life by comparison with the new ones. I'm not sure why the new pads had the weird cutout at the leading and trailing edges; possibly some sort of anti-squeal hack.

The old pads had also developed a little conformity with the rust ridges on the rotors, which would probably cause most shops to insist on brake lathing [at significant extra cost]. For what purpose? To expose more fresh metal to the elements? Bah.

Wear indicator doesnt fit on outer side The pad with the wear indicator needs to go on the inner position because it won't fit under the caliper frame on the outside when the pad is fresh. The wear spring wants to be on the normal trailing edge because if it ever does make contact with the rotor, we would want the little bits of metal shrieking off to fall away without getting sent through the rest of the works.

Not retracted enough yet All that piston-pushing still hadn't opened up quite enough space ...

Finally open enough to fit ... but a little more persuasion sent it back pretty much flush with the caliper edge and it could finally accomodate the fat sandwich of all the parts in between.

The rest of the job was just like the usual yearly cleanout, making sure the slide pins were correctly lubed and running freely, etc. While the edges and vent channels in the rotors looked pretty nasty by this point, there was nothing wrong with them and the areas that really matter were just fine. By keeping all the various mating surfaces clean and lightly greased on occasion I've never had the trouble with removal and reinstallation of wheels, rotors, or anything else that many others seem to report as problematic.

I didn't even bother investigating the bleed plug on the second caliper, because the pushback method had been quite easy and I *didn't* actually have a fresh bottle of DOT3 brake fluid on hand so why waste what was in here [and then have to dispose of the remains correctly]. The squeeze went even faster on the second caliper, now that I had a feel for the force needed. After finishing with both sides I noticed that the fluid in the reservoir, which had been been somwehere between the lines, was right back up near the "F" mark.

Next thing was to go for a little drive and start bedding in the pads to better match the rotors. Having the accumulator relays still out left me on the "tertiary backup" purely unassisted hydraulic path to the front wheels, so I wouldn't have great braking power but would also be able to directly feel whatever was going on up front. It was enough to carefully tool around the neighborhood with light pressure on the brake pedal and get an initial level of settling done, aka "redneck rotor turning". That process had the rust ridges fighting with the inner and outer edges of the new pads, making a fairly hideous grinding sound and getting the rotors plenty hot -- probably some of the lowest-MPG driving I've ever done. Like totally channeling that Sikes douchebag. With no assist and limited contact surface for a while, actually *stopping* required quite a lot of foot pressure and I kept thinking about how spoiled we all are with modern power brakes.

For the performance-braking enthusiasts horrified by reading this, think about the grindy noises we hear/feel every time our brakes sit in the wet and get surface rust and then we go drive on them. Between rust and road dirt our pads will frequently have to deal with various unevenly-distributed abrasives, and that's just life. They're designed to handle it. Really, for the best brake longevity try to simply avoid using them as much as possible.

Eventually I stopped and put the relays back in to pressurize the system back up. After the usual four power-up reboots to self-clear the various errors I did some cautious testing in a parking lot to make sure full assist and force-distribution were restored, and then everything felt completely normal again. I expected the 8 MPH stopping transition from regen back to hydraulic to be rather rough until the pads finished bedding in, but from inside the car I couldn't tell anything had changed. Well, other than the pressure read being a little more flickery on the fronts than previously, but I expected that would continue to settle down over time.

So, all set for the next down-the-mountain screaming adventure, trailing a cloud of brake dust!

_H*   130726