Experiments In Brake Fluids

[Hugh Janus]

Kevin Cameron (Robert Martin/)

Years ago I worked with a man who had exceptional skills in the “mechanic arts”—machining, welding, fabricating, and imaginative design. He taught me a lot. Although a graduate of a respected trade school, he had learned most of what he knew in the course of his work, finding practical solutions to experimental problems encountered by university researchers.

Early in his extensive self-education he built himself a classic hot rod. Having spent about everything he had on this creation, he was not about to spring for brake fluid. He filled and bled the rod’s brake system with kerosene and set off for school.

“Ha!” he thought to himself as he rumbled along. “Brakes work fine.”

At the school he parked and went into class.

Returning to his car at the end of the day he got in, started up, and upon trying to drive off found he could not. All four wheels were locked solid.

The “innovation” of using kerosene instead of brake fluid had revealed why brake fluid exists: the ethylene propylene rubber (EPR) of which brake system seals and cups are made is hydrocarbon in origin, so the hydrocarbon we know as kerosene has a strong affinity for it. As the EPR absorbed the kerosene it swelled. Swelling of the master cylinder’s piston seal first blocked the return port (which normally lets fluid return to the reservoir when the brakes are released) and then its continued expansion and that of the piston cups in the wheel cylinders (his was a drum-braked car) applied the brakes and held them that way.

Ethylene propylene rubber (EPR) seals will swell when subjected to hydrocarbon-based fluids. (Motorcyclist /)

The entire system had to be disassembled and cleaned in a solvent compatible with EPR (polar solvents such as isopropyl alcohol—not hydrocarbons such as kerosene, gasoline, et cetera). Then every rubber part had to be replaced with new. It was a labor-intensive practical lesson.

Brake fluid usually consists of a low-viscosity glycol ether liquid that remains easily fluid even at sub-zero temperature, plus a lubricant to allow easy piston movement. There is a compromise inherent in meeting that low temperature fluidity requirement; fluids that retain low viscosity down to low temperatures tend to have lower boiling points. Glycols are alcohols and so attract and retain water, which at sea level boils at 212 degrees Fahrenheit. The moment you open a tin of brake fluid, it begins to absorb water from the atmosphere (stop breathing on my brake fluid!) and its boiling point falls.

Once a container of brake fluid is opened, water will begin to be absorbed and the boil point of said fluid begins to decrease. Think twice about using that old, open can of fluid on the shelf. (Motorcyclist /)

Brake calipers are designed to keep frictional heat from the pads from reaching the fluid, but get the fluid hot enough and it can boil. As anyone who has ever bled brakes can attest, even tiny bubbles of gas in brake fluid can make the lever spongy. Boiling fluid can push itself back to the master cylinder reservoir, leaving you with a lever that comes to the bar the next time you brake. This is why motorists are advised to replace their brake fluid with fresh at two-year intervals, in order to prevent a steady fall in fluid boiling point from absorbed water.

This is also why Brembo’s latest calipers have their distinctive open design, which encourages the airflow that can reduce caliper operating temperature.

Back in the 1970s I read about DOT 5 silicone brake fluid, which does not absorb water and has a reliably high boiling point. Gotta have it! Returning from the auto parts store with the miracle fluid, I flushed and filled my system. Hmm, that’s odd—the lever doesn’t return. The more I cycled the lever, the more I realized it wasn’t getting better. Then I pulled out my trusty copy of Gunderson and Hart’s Synthetic Lubricants. They didn’t want to step on the toes of the giant chemical firms, but it was clear from the data presented that silicones are far from first choice as lubricants.

Okay, lesson learned. I again drained and flushed my brake system, then refilled with the DOT 3 fluid I had previously been using without problems more serious than its demonic ability to wreck expensive paint jobs.

Source