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I recently saw a Shimano hydraulic disk brake system that was about to be bled with yellow oil in it (Shimano's mineral oil is red). Most likely it was third party mineral oil like Finish Line's yellow mineral oil or else the calipers would have probably started to leak quickly. Still, I would have liked to confirm. The reverse situation where mineral oil put inside a DOT brake system could also happen with less drastic damages.

Before doing a brake bleed, is there a way to quickly and reliably identify if the oil already present in the system is mineral or DOT without solely relying on its colour?

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    Without a chemical o rheological laboratory test? Hardly. Feb 11, 2023 at 15:56
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    @VladimirFГероямслава I know mineral oil is hydrophobic while DOT absorbs humidity so I thought maybe there could be something to do like mixing water (or something else) with them in a small recipient and observe a different mixture behaviour. There may be other properties differences I am not aware of that could lead to such simple experiments, who knows.
    – olliebulle
    Feb 11, 2023 at 16:10
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    "One benefit of mineral oil over DOT fluid is that it is non-toxic." -- Not the route I would go, but there is that (LOL). I like the idea of putting the old brake fluid into a clear container and adding a drop or two of water. Mineral oil will not absorb it (it remains as a drop at the bottom) whereas DOT 3/4/5.1 will absorb a small amount. If the water drop eventually gets absorbed, you are pretty good to say it is DOT fluid. The key is using just enough water to be able to detect it, and not so much that you would not be able to tell if any was absorbed.
    – Ted Hohl
    Feb 11, 2023 at 18:09
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    So you do not want to distinguish different brands of mineral oil? Feb 13, 2023 at 14:15
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    @olliebulle I recently drained some very old brakes and the Shimano fluid had lost its red color and looked slightly yellow.
    – Paul H
    Feb 14, 2023 at 4:27

2 Answers 2

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So I tried out myself a little experiment to determine if it was possible to differentiate mineral oil from DOT oil without relying on its colour.

Hypothesis

Since mineral oil is hydrophobic and DOT oil absorbs humidity, it should be possible to mix some water with them and observe a different behaviour.

  • Water drops put in a recipient containing mineral oil should not mix with it and reach the bottom of the recipient.
  • Water drops put in a recipient containing DOT oil should be absorbed by it and not reach the bottom of the recipient.

Experiment

  1. Safety first.

enter image description here

  1. Gather new mineral and DOT oil.

enter image description here

  1. Put some mineral oil and DOT oil inside a transparent recipient. Get also some tap water inside a small bottle.

enter image description here

  1. Put a few drops of water into each of the recipients.

Observations

  1. Water drops put into the mineral oil recipient go to the bottom quickly and keep their shape (1st picture). If shaken a bit, they regroup (2nd picture).

enter image description here enter image description here

  1. Water drops put into the DOT oil recipient do not go to the bottom (1st picture) but rather get mixed with the fluid at the surface (2nd picture).

enter image description here enter image description here

Conclusion

It is possible and pretty easy to figure out if an unknown brake fluid is mineral oil or DOT oil by mixing them with some water.

Bonus

We can also add much more water and observe something interesting. All the water is immediately going at the bottom of the recipient with mineral oil and completely mixes with DOT oil.

This is a picture right after adding a good quantity of water:

enter image description here

This one shows the same recipients but 6 hours later. Not much changed for the mineral oil but we can see that the DOT oil has had time to absorb the water and that little bubbles have appeared:

enter image description here

Edit

MaplePanda suggested (thanks!) to do the experiment the other way around by putting drops of oil into water instead. This is a good idea so I tried it and here is the result.

The recipient on the left is the one with a few drops of mineral oil. We can see a thin red layer on top and nothing elsewhere.

The recipient on the right is the one with a few drops of DOT oil. We can't see any yellow layer on top but rather tiny bubbles in the lower half portion of the recipient.

enter image description here

This method is as effective as the previous one at detecting the type of oil but is simpler as you only need a few drops of oil. Of course, I could have used way less oil in my first experiment, but still, I think this second method is superior.

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    Any chance you could test this experiment the other way around (as in, adding a few drops of the unknown fluid to a container of water)? The downside of your current setup is that you need a relatively large sample of the fluid.
    – MaplePanda
    Feb 15, 2023 at 2:42
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Safety first - I'd web-search the caliper's model number and see what fluid it is supposed to have.

Then I'd flush the whole lot out and put in the correct fluid. Work the action a bunch of times, then flush THAT out and replace with the correct fluid. The old fluids get disposed of properly at the correct local facility.

Two possibilities exist:

  1. The fluid was correct originally, you've put in the same stuff and now have good brakes with clean fluid.

  2. The fluid was wrong and you've now put in the correct stuff. In the second case, the wrong fluid may have damaged seals over time. You'll know that if it leaks, in which case you're up for replacement seals and a rebuild, or replace the whole system.

There's normally enough fluid in a 50 mL bottle to completely refill both brakes once. No, its not cheap but its not expensive compared to brake failure.

For peace of mind, consider adding a permanent label somewhere that won't wear off saying "Finishline yellow MINERAL OIL 2023-02-14" or something to that effect, to show what you put in and when it was done.

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  • This is great real life advice thanks. The brake system needs mineral oil, but I was just curious if there was a way to tell if an unknown brake fluid was DOT or mineral with a simple test. Someone could have put the wrong fluid in but I guess my question is more theoretical than practical.
    – olliebulle
    Feb 14, 2023 at 1:25
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    @olliebulle Its pretty unlikely someone's used the wrong fluid. If the brake is functioning correctly, and only bike shops or decent home mechanics worked on it, then the fluid's probably correct.
    – Criggie
    Feb 14, 2023 at 3:41
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    @olliebulle your other option is to ask the owner of the bike - they may say exactly what's in it, or that "it was serviced at LBS Bikes in my hometown" which is also useful. If they say "my cousin bubba fixed it up real good" then check the bike closely for workmanship.
    – Criggie
    Feb 14, 2023 at 3:44

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