Bottom Bracket Selection Guide
Hydraulic Disc Brake Bleeding Done Right (Shimano)
I know, I know, we're known for manufacturing the most efficient bottom brackets in the world, but we here at BBInfinite are skilled technicians with years of experience and loads of time-saving techniques that we want to share with everyone. Bleeding a hydraulic brake system isn't hard, but it is easy to screw up. We show you how to do it step by step to avoid the drama. Bottom line: It's easy when you know how.
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 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 bleed 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.
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