Hydraulic Disc Brake Bleeding Done Right (Shimano)


Gary with BBInfinite. Most of us have
experienced this…a dead lever. And as you know it means that this break needs to
be bled because air has gotten into the lines and that leaves this thing spongy
or just plain old not functioning. I’m about to show you how to bleed this
brake system but first I’m going to briefly illustrate how the hydraulic
systems work in principle so that you will have the confidence to tackle this
project. And if you’re not the kind of guy that wants to tackle this project at
least you will be will be an informed customer, and you will get the opportunity to be a
know-it-all at your bike shop when the mechanics bring this up. To demonstrate
an important hydraulic principle about what’s going on inside of this lever
when you pull that lever back is that the cylinder inside that is supposed to
push the fluid out so that the pads can then contact the rotor, is getting spongy
because there is a volume of air inside of it. Now, air compresses as we all know, which is why scuba gear works and any number of things—compressed gases—and fluid does
not; so when there is air in there it’s going to give us this spongy effect
because it is acting like an air spring. In order to do this job you need some
materials and tools and as always you need some clean shop rags, a personal
favorite, White Lightning Clean Streak. Every shop should have a can of this
stuff it is indispensable for degreasing and cleaning things after you’ve been
working, and you will need some tools that go along with whatever brake system
you’re using. Some will require an Allen wrench of some kind, a hex, and then they
will use an open end wrench for opening up fittings. Once again it depends on
what you’re using, what manufacturer, and that brings us to you will also need
some kind of brake bleed kit. This particular one was purchased off of
Amazon for like 20 bucks, prime shipping, gotta love that, and you’re going to need
that and also one specific to your system if you can get it. And last but
not least and probably the most important, you have to know what kind of
fluid your system uses. If it uses dot fluid dot 5.1 dot 4.0, or like Shimano,
Magura, and Tetro, and many others, they use mineral oil. You have to use the
specific one for your system. If you do not you are going to destroy it. And I
have to stress that, if you put mineral oil in a dots system, and vice versa, your
brake system will fail because the seals will be compromised, they will swell, and
your systems will completely not work. Now that we have all of the tools and we
have the right fluid let’s begin. Next, we’re going to press the syringe and a
lot of these kits are going to come with a really long piece of flexible hose. You
want to cut it down so you’re not having to deal with this big thing whipping
around it only needs to be long enough to reach the bottom of the fluid bottle
and also on the fluid bottle I like to get small quantities rather than a big
old jug, and the reason for this is all of these fluids are hygroscopic meaning
they actually take moisture out of the air and it enters into the fluid now
this causes a problem later on it could call the problem brake fade for sure as
one what when water is in your system and it’s exposed to very high
temperatures which is highly probable in these brake systems that work on
friction that it’s going to boil that water it’s going to turn into steam and
then it is going to basically make your system go just as spongy as if it was
full of airs so at this point we’re going to go ahead and draw fluid up into
the syringe making sure that plunger is pushed in all the way and we’re going to
draw the fluid in very slowly and I mean very slowly and the reason for this is
that especially this mineral oil it tends to aerate very easily so if you
just pull up the syringe very quickly it’s going to swirl a lot of bubbles
into it and now these need to leave need to be out of here before we can use it so what I’m going to do is I’m going to
go ahead and push back a little bit come up to the top
see the bubbles come out just like that basically bleeding the syringe we’re
going to leave the system with because air is the enemy here and then I am
going to sit here and wait for these bubbles to rise to the top so I can
chase them out of here make sure there’s not a single bubble inside of this
syringe before we move on the next thing we want to do in our prep is we want to
level this lever and the reason for that is the fluid in the reservoir inside
needs to be perfectly level just like if you were trying to fill a glass all the
way to the top we’re trying to make sure no air can get in and then we’re not
going to introduce any more air so we don’t need to make sure that the fluid
can take up the entire space all right now I’m going to go ahead and put this
little cup on the top this is the way Shimano wants you to do it so I’m doing
it their way oh also a lot of these systems they have little o-ring inside
don’t lose that it could be bad many other systems such as Ram wants you
to screw in a fitting up here and use a syringe to push and pull from the front
and the back Shimano is not that way we’re going to
be forcing fluid only from one direction from the caliper to the source up here
in the fluid will be pushing up so keep an eye out as we are bleeding because if
you push up on emu fluid up in here you can have it running out now we need to
add just a little bit of fluid to it so that we just have a good starting point
in form and basically an airlock a fluid inside it’s a small quantity let’s do it now that we’ve attached our little
reservoir we need to go in here and depending on what kind of lever you have
you need to back off any adjustments that have ever been made to it what I
mean by backing it off if you’ve ever turned a screw on the lever to make the
lever come further out you need to bring it as far in as possible and the reason
for that is you’re actually pulling the piston back inside of the reservoir here
and you’re going to allow the maximum amount of fluid to enter the system and
that way with that nice displacement you’re going to be able to get your
adjustments very crisp when this job is all done remembering what I said about
we need to try to visualize what the air and the fluid are doing in these systems
we need to look at this rear caliper this is particularly problematic because
the bleed port is actually at the lowest point on this caliper it needs to be at
the highest point in order to put the air to be able to come out so we need to
remove it and orient it in such a way that allows that to happen let me show
you what I’m talking about basically what we’ve got going on is the caliper
is positioned in such a way that air bubble is trapped in the bleed port is
at the bottom what we want is we need to fluid to push the air out of the system
right away so we need to Orient the caliper in such a way that the air
will be pushed out first and the bleep port or in the case of this Shimano the
line is that the highest point and we’re pushing fluid from the bottom as you’ll
see in just a moment this will force all of the air to the reservoir where it
will safely go out of the system and be displaced by fluid another important
thing in a job that has a multitude of important thing is to remove the brake
pads from the calipers anytime you are bleeding the system or you have brake
fluid anywhere near it and there’s a very good reason for this and I mean a
very good reason you are going to ruin a set of brake pads in a heartbeat if you
don’t get them out of here because you get even the slightest amount of fluid
on these pads they are going to squeak and they are done for there’s no way to
fix it without buying another set of pads the next thing to do is install a
bleed block and you have to sometimes it’s hard to get it in there you’ve got
to force the piston into the bores as far as they will go so that when you
bolt it back on the bike and you have the center of the rotor in between the
pads you have the maximum amount of throw in the piston is going to give you
the correct air gap between the pads and the rotor alright next we want to do is
we want to take this little rubber cap completely off of the caliper and set it
aside don’t lose it alright we need to make
sure that the fluid is pushed all the way to the tip of the hose before we
attach it to the little port on the caliper now we have the syringe attached to the
port which is appropriate for this system this is very specific to each
system some of them you’ll attach it directly to them to the bleed screw just
like you would on an automotive style caliper Shimano has recently gone
through a system like this where this screw actually blocks a channel in here
and opens that up now we want to orient our caliper to chase the air out now
I’ve been stressing about to get all the air out get all the air out but then you
see this little bubble this is the inevitability here you’re going to get a
little bubble because there is air trapped just in this little bleed nipple
on the caliper so we just want to push that through right away it’s going to go
right to the cup a lot of people want to draw back on it and they will draw that
bubble up into the syringe I don’t think that’s necessary and it can be
counterproductive because very often you’ll draw this little bubble out and
here comes another one and another one and what’s happening is you’re drawing
air into the system from downstream so just push it out right away now with the bleed block in place crying
out loud have the bleed brought block in press play I can go up there and I can
squeeze the lever if you squeeze the lever without this
bleep lock in place it’s going to blow the Pistons out and you’re going to have
a very bad day there we go hard as a rock and that’s
what we’re looking for now the bleeding process is complete and that’s proof
positive and I like to go ahead and take the syringe and draw the fluid out of
the cup as much as possible so when I pull it off I don’t have fluid spilling
all over the place now if there was an o-ring
under the little bleed screw or anything like that
put the o-ring back on if you don’t when you push this lever you could have some
fluid bubbling out of the top so you know if you have some fluid bubbling on
the top you should look on your bench probably an o-ring sitting on it now
these are very small fasteners okay and we’re only trying to tighten it tight
enough to compress the o-ring if a reason why they put a two and a half
millimeter allen on these they don’t need to be tight and gorilla tight just
enough goes a long way all right of course I want to get these
things back where they’re supposed to be nice and even break feel is good
actually want to adjust this one a little bit now it’s a little bit too
hard we need to have them we need these things to feel nice and even and last
but not least all of the fluids must be disposed of properly for the sake of the
environment well there you have it we’ve done it we’ve bled the brakes
we started by prepping properly making sure that our syringe was prepped and
that the levers were open the pistons were pushed all the way in step by step
we made sure to chase the air out orient the caliper properly everything that was
required and we’ve got the job done the first time the right way subscribe
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