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In case you don't have 'original' bicycle specific brake fluid on hand (e.g. "Magura Royal Blood"), what might be a good alternative to use (in a pinch) in hydraulic (disc) brakes? (for brakes that take mineral oil).

How long would such an alternative last? Which problems might occur and how soon?

I've read a few suggestions for alternatives:

  • Fork cartridge oil (the lighter (more viscous) variant)
  • Total / Castrol LHM Plus Fluid ( I believe it's power steering fluid)

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Questions:

  • What are some alternative (/better) suggestions for alternatives?

  • Would the Total/Castrol LHM Fluid or fork cartridge oil be a good alternative?

  • Does anyone have experience with using either a fore mentioned options for brake fluid?

  • Would the heat generated by braking be an issue for either of these/other alternatives?

What are the requirements for a good braking oil/fluid?, my thoughts:

  • High boiling point

  • high fluidity

Do you agree with these requirements? Did I miss any or are some of them simply incorrect?

Any suggestions are welcome. Thank you!

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    Starter: I've heard of Baby Oil being used as a substitute for Mineral Oil, but I have no experience. – Criggie Feb 4 at 3:04
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    The question is based on a lot of poor information and its premise is entirely flawed so it cannot be answered. LHM is widely discussed as a suitable, readily available alternate to Mineral oil. Mineral oil does not absorb water. If you are getting 'pockets' of water inside your mineral oil brakes, you got bigger problems than a bit of boiling water. DOT's boiling point lowers as it absorbs water, but it also does not get 'pockets' of water. – mattnz Feb 4 at 8:16
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    A nice article giving info about different types of brake fluid: epicbleedsolutions.com/blogs/articles/… – Andy P Feb 4 at 10:28
  • @mattnz thank you for your suggestions! :) I don't see how the general question "what are some alternatives to bike specific braking fluid " cannot be answered. Especially since I already got some suggestions. The premise (regarding water absorbtion) might very well be incorrect as you state, I got that from the forum post regarding using fork cartridge fluid as an alternative (it is linked in the question) and I explicitly asked in my question if someone could confirm this info to be true or false. If you could mention what else is wrong with my question I can try and improve it, thanks – Maarten -Monica for president Feb 4 at 12:05
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    +1 on the baby oil front for oil based systems - youtube.com/watch?v=63VIuPiX3CA – Warren Burton Feb 4 at 15:10
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The rubber seals in mineral oil levers and calipers are very easily damaged by using the wrong fluid, and should this occur replacement seals aren't typically available. Experimenting with alternative or in-a-pinch brake fluids is a bad idea for this reason. The results are unpredictable. In the worst case, a brake failure could occur.

Running a different viscosity than intended can also alter brake performance. In particular, higher viscosity fluids have been known to sometimes cause either slower or incomplete piston return. Of course the specifics of the hose, brake, piston count, pad spring, etc are all going to play into that, a fact that no meathead MTB forum poster is ever going to consider for a hot second before telling you to go put LMH in your brakes just because they did it and it didn't blow up.

There's a type of DIYer who loves talking about how they've had their bike set up with hack X for Y years and it's totally fine, so it of course will always be fine for everyone forever. That's a good type of person to categorically ignore. Most of the time, their shadetree nonsense doesn't work as well as they want to believe, and they're really only out to appear as though they have something meaningful to contribute, when in fact the opposite is almost always true.

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  • +1: Especially like the advice in the last paragraph. Very wise. – mattnz Feb 4 at 19:21
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Seth from "Seth Bike Hacks" youtube channel did a test recently with a baby oil, including disassembling the caliper and checking the seals for damage under a microscope. Full video here.

Also I remember reading in a review of Shimano brakes on bikeradar that in a post-crash pinch they used olive oil and got down the mountain OK. But this is rather an extreme example.

The only substitute I have a personal experience with was using sewing machine lubrication oil in very old Magura brakes as a long-term replacement. It did not seem to damage the brakes, but for some reason required bleeding the breakes much more often. It is possible that it was slightly out gassing under temperature. This is also what I would be most afraid of when using replacement - brake fluids are specifically engineered to avoid that, and it leads to mushy feeling on the lever.

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  • Since brakes may decide over life or death I believe the best alternative is to use the correct fluid. You wouldn't replace brake cables with fishing line. – Carel Feb 5 at 14:29
  • @Carel, the main difference here is that baby oil is mineral oil. – JPhi1618 Feb 5 at 17:07
  • I'd add that per Wikipedia, mineral oil is "... any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum ..." That implies that different types of mineral oil could have different chemical formulae - higher alkane simply means 9 or more carbon atoms, and nonane through hexadecane (16 C atoms) appear to be liquid at standard conditions. So, to @JPhi1618's point, baby oil can be both a mineral oil and yet be different from Shimano's oil or Magura's oil. Different mineral oils might have different chemical properties. – Weiwen Ng Feb 5 at 23:07
  • That said, I'm slightly amused by the anecdote about filling a line with olive oil in a pinch. I noted in some quick research that Shimano mineral oil has a manufacturer-reported boiling point of 280 degrees celsius (536 Fahrenheit). The boiling point for olive oil is reported to be ... 570F / 298C, so pretty close on that parameter! (NB: if you Google this, you'll see that the smoking point for olive oil is considerably lower, but that describes the temperature when oil exposed to air starts to smoke, so not relevant in a hydraulic disc scenario). – Weiwen Ng Feb 5 at 23:10
  • @JPhi1618: Really, look at the composition of these cosmetics: mostly water. Would you put engine oil (mineral oil) on your little one's bottom? And if there's water it will boil and produce steam. – Carel Feb 6 at 18:59

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