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I want to learn to bleed my new disc brakes. While there is a wealth of knowledge on the topic, it is difficult to tell the difference between pedantic explanations and sloppy work.

In the official Avid bleeding instruction video a meticulous procedure is explained, of course using their proprietary kit.

Shimano disc brake bleeding seems much simplified, and the kit is different (simpler).

Questions:

  • The avid kit looks really cool, with the clips on the hoses. And is not very expensive, at $60. Can it be used to bleed any kind of brake? With an adapter?
  • The avid instruction stresses removing any air from the liquid prior to the bleed (they clamp the hose of the syringe and pull on the piston). Why is this not done in the Shimano video? I went to a (good) mechanic with my Shimano BR-M395 and he didn't do such a thing. He didn't even unscrew the caliper fluid bolt - only operated on the lever.
  • Is Magura Mineral oil fully compatible with Shimano mineral oil brakes? Can the two kinds of oil be mixed?
  • At the LBS they told me that the procedure is complicated with Avids, because their caliper design is awkward, easily trapping air. On the other hand, Shimano brakes are expected to fill up properly and without air by just tapping the hose while filling. Is this true?
  • Any other differences between the procedures for Avid, Shimano, Magura, Tekrto, Clark's?
  • Is it true that the oil is very dangerous for the skin and the eyes, and gloves must be worn?
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2 Answers 2

Different brakes use different designs and different fluids, and as such have slightly different bleed kits and procedures. I've answered what I can below.

The avid instruction stresses removing any air from the liquid prior to the bleed (they clamp the hose of the syringe and pull on the piston). Why is this not done in the Shimano video?

I suspect Shimano don't consider it to be necessary for their brakes. The fluid used in Avid and Shimano brakes is completely different, so I imagine it needs to be handled differently.

Is Magura Mineral oil fully compatible with Shimano mineral oil brakes? Can the two kinds of oil be mixed?

In general, yes, different brands of mineral fluid are compatible (including the stuff used in Citroen cars). If you switch to another brand of mineral fluid, it's probably best to flush the brake entirely, rather than just bleeding. however, DOT fluid cannot be used in a Mineral fluid brake, and vice-versa. DOT will eat through the seals used in Mineral brakes.

At the LBS they told me that the procedure is complicated with Avids, because their caliper design is awkward, easily trapping air. On the other hand, Shimano brakes are expected to fill up properly and without air by just tapping the hose while filling. Is this true?

I don't know about the current Avid brakes, but Avids do have a reputation for being really difficult to bleed properly. The older levers also trapped air above the bleed port.

For current Shimano brakes I use a syringe and push fluid through from the bottom to the top when bleeding them.

Any other differences between the procedures for Avid, Shimano, Magura, Tekrto, Clark's?

Yes, but you'd need to refer to the various manufacturers documentation.

Is it true that the oil is very dangerous for the skin and the eyes, and gloves must be worn?

DOT4/5 fluid is horrible stuff. Mineral fluid is less nasty. In both cases you don't want it anywhere near your eyes and should probably avoid getting it in your skin. But the warnings you see in videos and on literature are largely a legal thing: They don't want you to be able to sue them if you blind yourself!

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150ml Magura mineral oil is 12BGN, while I got the whole brake system for 50BGN. Seems like a waste to throw away all the old oil. What problems could occur if I don't do this? –  Vorac Feb 18 at 15:42
    
If the oils are of a different consistency (e.g. if Magura oil is thinner than Shimano) they'll separate in the hose (the heavier one will sink to the bottom of the system). I can't imagine that would lead to good braking! Bicycle disc brakes use hardly any fluid anyway: I've seen people on other forums say they've filled a pair of XT brakes with a 50ml bottle of Shimano fluid! –  Olly Hodgson Feb 18 at 16:23
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By the way, for Shimano brakes, this looks like a very good bleed kit: epicbleedsolutions.com/products/bleed-kits/shimano/complete-kit. There's also complete bleed instructions on their site. –  Olly Hodgson Feb 18 at 16:25
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In any case, brakes are one place where you probably don't want to be stingy with money... Another thing to note that DOT 4 and 5.1 can be mixed (you can probably even use DOT 3), but DOT 5 is totally different than DOT 4, 5.1. –  Batman Feb 18 at 16:46

It's necessary to understand the design of the systems to understand why Avid requires a separate bleed kit.

There is a significant difference in the design of the brake system between Avid and other manufacturers such as Shimano and this is the main reason on why Avid have a proprietary bleed kit.

Although an 'open' hydraulic system Avid's Taperbore resembles a 'closed' system, one that operates in a vacuum. The Avid system uses a small reservoir fixed amount of fluid in the system and a bladder around the master piston with air in it. Shimano and other brands use a larger reservoir with a bladder separating the brake fluid (in this case mineral oil) and air. In both cases air is used to compensate for the fluid in the system expanding.

Because the Avid system operates in a vacuum it requires all air to be removed from the system and that is why you need the two syringe set, other designs simply require fluid to be topped up in the reservoir and air worked out.

All the different brake fluids have different characteristics (boiling points, freezing points, amount of expansion etc.). Avid specifies DOT 4 and 5.1, which is a pain. Like all solvents you don't want to get it on your hands but it isn't going to kill you unless you work with it everyday without protection. DOT fluid is hygroscopic, it attracts water so although you can buy it in larger quantities cheaper than through Avid it doesn't have a good shelf life once opened and probably shouldn't be opened again.

To bleed Elixirs you don't need a Avid bleed kit, just a hose barb that fits and some generic hose. I use the brass hose barb which is for my Rockshox x-loc and generic hose and syringes.

Also you can do a less complicated 'speed bleed' to remove air from the system, but a full bleed is better. I bleed the brakes on my hack/ commuter but leave it up to the wrench at my LBS to bleed the brakes on my XC bike (I just don't see the point in paying a few hundred for good brakes and then not getting them optimised by a professional).

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