Prius brake maintenance
or, the benefits of regenerative braking...
As part of the springtime-and-50K service, it's useful go around to all
four wheels on the Prius and inspect the brakes. Maybe the tires need to
be rotated at this point too, which would also provide a good opportunity.
With proper use of regen braking, there is minimal wear and generation of
brake dust, so there's relatively little to do!
Use the linked larger pictures to clarify anything, particularly any
If the car was just powered up, wait at least two minutes after power-off
for the brake system to go through its shutdown self-test [part of which
involves repressurizing the brake lines]. Then, pull these two relays in
the under-hood block. Pump the brake pedal slowly, holding it down at the
pushed position for a short time, until that little high-pitched beeper starts
going off under there. This mostly depressurizes the hydraulic system and
disables the electronic control part thereof, and ensures that it's not going
to self-test by itself while you've got things apart. Simply opening the
driver's door brings the brake system online, and you don't want the hydraulic
system suddenly pushing the pistons at 2000 PSI -- either right out of their
cylinders, or onto your fingers.
We'll start on the front wheels.
The two main caliper bolts [orange] are torqued fairly tight, and for the
upper one the strut bolt end is just a *little* too in the way to get a socket
in there. The best option for these is a box wrench and a fair amount of
grunt. [Spec is 81 ft-lb.] The slide-pin bolts [blue] are easier to deal
with -- but take note:
If the pin bolts bind in the inner pin threads at all, you'll wind up spinning
the whole pair. Use an open-end to apply opposing torque to loosen, until
you're sure the bolt can freely come out. Try to not spin the pins themselves
too much yet, until you know what that's doing inside. That's part of what
this article is about, in fact -- slide-pin paranoia.
Separating the cylinder assembly from the rest of the caliper allows hanging
much less metal up out of the way -- it's aluminum, and very light. While
simply hooking it over the coil spring may be able to hold it up, it's prudent
to tie it off so it doesn't drop and yank on the brake line. At this point,
the pads and all the backing plates and clips can be removed. Now there's
less risk of banging into delicate things while huffing away on the main
Once the main caliper frame is off, the disk should slide right off the hub.
All the parts can now be laid out for cleaning and inspection. Don't mix up
the various anti-squeal shims -- they're similar, but have subtle differences.
You can use wear patterns on them as hints if needed. Not much to be done
about the rust inside the rotor and ventilation holes; this is New England...
At 52,000 miles, still *plenty* of meat left on these pads. They'll probably
still look like this at 100,000, too, except with more rust.
Now we turn our attention to the slide pins. These should be maintained
carefully, because if they freeze up with corrosion, the caliper can't
self-center and brake pad pressure becomes uneven and starts creating undue
pressure on the wheel bearings. First of all, try to turn the hex-nut portion
of the pin with your fingers, watching for boot distortion. You may feel a
sort of "releasing" or "breaking loose" sensation as the pin begins turning,
and then it turns much more freely. We will see why in a moment.
Pull the lip of the pin boot back, either with your fingers or [carefully]
with a tool, to release it from the flange...
and the pin can pop right out, bringing a bunch of grease with it. Try not
to let dirt get into the grease. For reference, the Toyota service manual
refers to using "Lithium soap base glycol grease" here.
Carefully tease the base of the boot out of its cavity, squishing it
slightly so the edge clears...
and remove it. Inspect the inside of the boot for grunge, and wipe off
the outside. If the boot is damaged it should be replaced, but I don't
know offhand if it's possible to order just those from Toyota. The manual
refers to it as a "non-reusable part", which is utter bunkum, but you'd
think parts counters might have them in stock.
Hey, there's some grease left at the bottom of the hole!
Hey, autofocus sometimes gets things comically wrong!
The spare grease can be carefully extracted and reused if it's clean.
Manual focus can be carefully used if your camera keeps screwing up.
The second pin shows an interesting wear pattern. The pins aren't round --
each one has three slightly flattened sides, in part to accomodate the grease
that's packed in around them. This one obviously wound up, purely by chance,
with this larger-radius portion pointing in a direction that allowed the
rest of the caliper to bear on it a little more. Now, most of the braking
stress is supposed to be taken by the pads bearing directly on the notches
in the caliper frame via those little spring-clip buffers, but pad plates
aren't made perfectly and a healthy amount of load can still be transmitted
to the piston assembly which floats on these pins.
This is why it's good to occasionally move the pins to a different position.
Turning them carefully not only brings a new stress surface in, it helps
redistribute the grease around inside. So even if they're not taken apart
on a regular basis, it's good to simply loosen the pin bolts, rotate the
pins around a little, check that the caliper freely moves on them but without
any grinding or excess play, and tighten them back up. That's an easy thing
to check at, say, every other tire rotation.
If we're taking things apart, we can rescue the extra unused grease from the
hole and possibly supply a little more, and regrease the pins. However, we
can't simply grease the pin and reassemble with the boot. Why?
The boot base hole fits snugly around the pin, and re-inserting a
heavily greased pin back through it like this would leave all the
grease inside the boot instead.
We can't insert the pin and then grease it, because the base of the
boot has to squish quite a bit to get it back into its cavity in the
caliper frame, which we could never do with the pin inserted.
So the answer is to use a small tool to deposit most of the grease near the
upper end of the hole, insert the base of the boot into the caliper frame,
and then insert the pin slowly by turning it back and forth to pick up the
grease and help distribute it down along the length. You should feel the
grease viscosity all the way down. Insert the pin all the way, until the
flange snaps into the top of the boot. There should also be a thin layer
of grease on the parts of the boot that contact the caliper frame or pin,
to help seal things.
If you've done the right things with the grease, there should be enough
in the pin flange to seal the outer lip of the boot too, such that when
you try to pull the pin back out a little the boot completely collapses
from the vacuum. However, don't leave it like this -- gently poke something
into the boot lip or just pull open one side with your fingers to break the
air seal, and move the pin to where the boot is about in its natural,
non-stressed position. This will be close to where it will sit when the
caliper is reassembled.
Reverse the dissasembly to get things back together, wiping things down as
needed. A little grease on those little spring-clip support plates that the
pads rest in can't hurt; the last thing you want is pads sticking in place and
then dragging the brakes. And a touch of anti-seize on various bolt threads
will make the whole job easier the next time around. Don't mongo the pin
bolts; in contrast with the main mounting bolts, their torque is only speced
at 24 ft-lb.
Now we move on to the rear brakes. There's even less to do here. Once the
wheel is off, the drum should just slide off, possibly helped by a gentle
tap or two.
If it's stuck onto the hub with rust, find just about any other bolt on the
car with a 12mm head [radiator support, perhaps?] and use one or two of them
in the smaller threaded holes to apply pressure to crack things loose.
The innards. Plenty of meat left on these shoes, too, which is a little
surprising because the rears are the ones that first take up what regeneration
This is the adjuster. The brakes should be self-adjusting, in that if the
shoes spread out too far during a braking event, a lever behind the adjuster
actually kicks the toothed wheel around a notch and automatically backs out
the screw so that the rest position of the brakes advances. The hydraulic
systems adapts to this, but the parking brake cable may need adjustment over
time to chase the shoe wear.
Nothing was really disassembled in this go-round because it all looks pretty
clean and the adjusters turn freely. Next time may see some re-greasing of
the adjuster threads and backing plate surfaces that the shoes rest against.
UPDATE: Further disassembly was needed a couple of days later, to get
(!) the answer to a question that came up about exactly how the self-
adjustment mechanism works. Details here.
On reassembly, just before bringing the lugnuts to finger-tight, it's good to
reach in and rotate the drum backwards [with respect to the car; this shows
the driver's side] to preload its hole play against the wheel lugs to resist
normal forward-moving braking force.
When done, remember to reinstall the ABS pump relays! You should hear the
accumulator run for quite a while next time the brake system is re-enabled.
_H* 070515 [and no, I didn't buy gas today either, but NOT because of
anything I read on the net...]