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RE; Brake Fluid


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Education requested please. Since the brake fluid we use in our bikes in cars is hydroscopic and one of the main reasons we are supposed to flush it, why don't they make the systems capable of using Mil-H-5606 which is not. Would Mil-H-5606 destroy the systems on a BMW bike?


Not going to use it just curious as this is the fluid required on my airplane.


Thanks in advance.

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It's better to use a hygroscopic brake fluid. If water vapor entered the system and wasn't absorbed, corrosive water deposits could accumulate and affect damage on some of the components of the brake system. The corrosive effect is less if the water is absorbed into the fluid in small amounts.

Perhaps it's less of concern with aircraft because maintenance schedules mandate regular fluid replacement; it's not so with motorcyles.

Provided that all the brake seals were compatible with Mil-H-5606, your bike may be fine with it when changed regularly.

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Mil-H-5606 is a mineral based hydraulic fluid and, as Haynes stated, D0T-3/4/5.1 are glycol-ethyl based and, thusly, are hygroscopic. Interestly, Citroen uses mineral based fluids (LHM) in their cars suspension and brakes. I believe the Mil-spec fluid can be substituted for LHM.

My step-father was a Citroen fan and I actually like(d) the quirky cars :thumbsup:. The DS-21 was a hoot to drive.


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"...Mil-H-5606 is a mineral based hydraulic fluid ..." and therefore not usable in any bike that uses DOT3/4 fluid. The seals aren't designed to resist mineral oil and won't survive.

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