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. |
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. |
All that piston-pushing still hadn't opened up quite enough space ... |
... 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! |