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DOT fluid or mineral oil?

30/05/25 21:56 Filed in: Gear
Car brakes always use DOT fluid. Why? Exactly because it is hygroscopic. Water will get into car brake systems during use, in some level, and if water was immiscible with the brake fluid, it would pool at low points in the system (probably the calipers) and start causing corrosion. Because it is absorbed into the brake fluid, the worst that happens is a decline in boiling point with age. It really only takes a small percentage of water in the fluid to really ruin the heat resistance of the system. Racing brakes get bled regularly because the excessive heat they are exposed to (glowing brake discs) mean that new brake fluid is important far more often than in a car used for driving around town at low speed.
I remember buying a particular brake fluid for my race car because the boiling point was considerably higher than standard fluid, but this brand was also inexpensive compared with exotic fluids with, perhaps, a few degrees more heat resistance.
 
On bikes, however, the water absorption issue is less relevant. It almost seems like a liability in DOT fluid brakes. Mineral oil is immiscible with water. If you get water in your bike brake lines it will run down to the lowest point. But there is no steel that I know of in any bike disc brake so no rust will ensue. That water, if it ends up in the caliper, will boil at 100C and the brakes will fail (steam is a very poor brake fluid). Mineral oil is actually a pretty good choice because the boiling point can be about the same as for DOT fluid, it doesn't harm paint (DOT is bad for paint), and it is generally benign on the skin (DOT and skin don't mix).
 
Shimano has used the same pink fluid since day one in their disc brakes. Magura has used the same blue fluid as long as I'm aware (but I don't know if those early Gustav brakes used it and I can't be bothered searching for the answer). Campagnolo brakes are current Magura brakes re-purposed to road use, and the mineral oil is very very similar to the Magura stuff (though, typically Campy, not identical). Hayes and Avid both used DOT fluid from their respective starts. Hayes is now part of the Manitou/Answer/Ringle/Sun brand range, and still uses DOT fluid. Avid is part of SRAM now and the AVID brand name is mostly historical. The most recent 3 new brakes from SRAM run on mineral oil, but everything before that was DOT fluid.
 
All my SRAM brakes use DOT fluid. I know I should bleed them more often than I do, but even the DH bike (which gets ridden on longer and steeper terrain than any other bike I have) has no issues with boiling brakes after 4 seasons on the original bleed (I shortened the hoses and swapped lever sides out of the box). In general, I bleed the brakes when an issue pops up.
 
Mineral oil and DOT fluid require completely different seals. So never mix them up. Even the bleed equipment has to be quarantined across fluids as the wrong fluid will ruin the seals in the syringes.
 
While most of the bikes at home are now SRAM with DOT fluid, there are still a couple of sets of Shimano brakes around. And I bled one bike with these just the other day (they had a LOT of air in the system and didn't work). I got really good at the gravity bleeding using a Shimano cup on the lever and catch container at the caliper. But this seems too hard after lots of Bleeding Edge bleeds with the SRAM tools. I re-purposed a Reverb seatpost bleeding syringe to go on the caliper end and push fluid up into the cup. I'm much happier with this arrangement as it is quick and easy and because the hydraulic actuator for the Reverb uses mineral oil the syringe is quite happy with Shimano brake oil in it (whereas the vet supply syringe I was using to extract oil from the bottle suffers from a swollen plunger - it doesn't like the oil at all).
Pushing the fluid in from the caliper and out the lever end is in the direction the bubbles want to go. Gravity feeding the fluid does eventually work, but it can involve some fussing around to clear all the air. I used to bleed my car brakes by hand, from master cylinder to caliper, but now I understand most mechanics use a vacuum bleeding machine that does it all quickly and perfectly. There's so much volume in car brake hoses that a little air is rarely the problem - it is burnt fluid and water-absorbed fluid that cars need to get rid of regularly.
 
The one advantage of DOT fluid is any car place can sell you a bottle and it is quite cheap because there are so many cars around. Mineral oil is specific to the brake brand being used and as any proprietary product, can be quite expensive. Having said that, I still have around 600ml of Shimano fluid remaining after so many bleeds over 15 years of disc brakes in the original 1 litre bottle I bought way back when. The 500ml DOT fluid bottles never get emptied by me, because it requires so little volume to bleed a bike and like I wrote above, I don't do it that often. (Note to self, buy a new bottle and then bleed them ALL in one flurry of activity, so they are all fresh.)
 
Ultimately I don't think any real advantage or disadvantage stems from the fluid type used once it is in the brakes. But for safer handling and longer storage life, the mineral oil wins. SRAM must have agreed with their recent switch.

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