I had some free time last month between work orders while I waited for our daily shipment of special order parts to arrive and wanted to work through a little experiment involving mineral oil in SRAM brakes. I have previously worked in a shop that strongly favored Shimano hydraulic disc brakes over SRAM’s offerings due to at-the-time frequent warranty issues and a tendency for DOT 5.1 fluid to go bad shortly after opening and, though my opinion has softened since then, generally mirror that sentiment.
DOT fluid is hygroscopic, meaning that it attracts water and therefore oxygen. Once the seal on a new bottle of DOT fluid has been broken, the fluid therein will absorb enough latent moisture from the air to make it nearly unusable in bicycle hydraulic systems in about a week. Most other hydraulic applications using DOT fluid (e.g. car brakes) are engineered to include a reservoir to which extraneous water and air can be diverted to negate this issue. In bicycle hydraulic brakes, this air reservoir is noticeably missing, meaning that no air can enter the system without compromising braking power. Hence the need for fresh fluid and an exacting bleed procedure that can be time consuming and often frustrating.
Though one can wax poetic about the lever feel and positive contact of Shimano brakes, from a mechanical perspective Shimano’s offerings shine because of their use of mineral oil. It is nontoxic, shelf stable, and shares effectively the same temperature range as DOT fluid. Perhaps most importantly, it is hydrophobic. While bleeding, instead of agonizing over the syringes full of fluid attempting to remove every last microscopic air bubble before bleeding, mineral oil rejects air and water and makes clean up the longest part of the process.
I take a lot of pride in backing the products we carry, but our brands primarily spec SRAM brakes on their bicycles. I was left with figuring out how to make them feel as good as I know they should. I could not help but wonder what would happen if I ignored The Forums and manufacturer’s specifications and used mineral oil in a brake spec’d for DOT fluid. I am no chemical engineer, so while I can understand in theory that using the wrong hydraulic fluid could deleteriously effect the seals of a brake, I did not have the appropriate background knowledge to understand that in fact. So I tried it.
We had a spare SRAM Level T brake on hand that I was able to test this theory on. Given that I was entering uncharted territory, I had no best practices to follow except what I could make up as I went. The system was flushed three times before the final bleed. The bleed procedure was a mixture of SRAM and Shimano’s recommendations that seemed most appropriate for what I was doing and took me thrice as long as usual as I ever so carefully went through step by step.
For the first fifteen minutes after bleeding, the brake felt great. Honestly, it did not feel much different than before, but anyway this was less about brake feel and more on longevity. I put it on a demo bike I was going to ride after work that night and parked it outside for a couple hours. The video below was taken after bringing the bike in to prep it to ride that night. It felt like the seals on and around the master cylinder in the lever had deteriorated, allowing fluid past the seals and seizing the brake. I did not feel safe riding it after that realization so the brake never encountered trail conditions, but it seemed clear enough that it was broken and not going to work. So if, like me, you take advice from The Forums and manufacturers cautioning about compatibility issues with a grain of salt, here’s your answer: mineral oil does not work in SRAM brakes.
As for what is best practice when bleeding SRAM brakes, I use a new bottle of DOT fluid every time. That is, at least, until SRAM reengineers their seals and tells me I can use mineral oil after all.