8

I was curious, given that fact that a lack of engine oil can completely destroy your engine, can a lack of brake fluid completely destroy your brakes? I can imagine how that could be even worst than a broken engine for safety reasons.

*What exactly are all of the disadvantages to running an engine when the brake fluid becomes too low?

  • 2
    Other than the brake booster's use of vacuum produced by the engine, the engine and braking system are totally independent of each other. – HandyHowie Jan 15 '16 at 21:18
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Not having brake fluid in your car won't destroy the brakes, but whatever you hit when the brakes don't work just might. :)

The brakes in basically every passenger car since the 50's are hydraulically operated. That means when you press the brake pedal you are pressurizing some fluid lines. Those lines run out to the wheels where the added pressure forces the pads against the rotors or drums.

Simplified brake system

If that fluid is low or missing you will end up with nothing but air in those lines. Air is very compressible, which means when you squeeze it it's volume decreases. The brake fluid in your car is for all intents and purposes incompressible. That means when your foot on the brake pedal operates the master cylinder which squeezes the fluid into the lines that force is transmitted almost completely to operating the brakes. When there is air in the line all you do is move the air around a little and you apply very little force into operating the brakes. When you have only a little air in the lines the pedal just feels "spongy" because you compress the bubbles down to nothing and then you are closing the brakes. When there is a lot of air, or no fluid at all, you just don't operate the brakes at all, and you have a very bad day..

The good news is that topping off the fluid with the right type (DOT-3, DOT-4, and a collection of others are used depending on what car you own) and then bleeding the brakes will bring them right back again. That assumes that when they were dry you didn't end up with any kind of debris in the lines. With no brakes you likely didn't drive it much though.. lol Bleeding the brakes just means opening those hydraulic lines at one wheel at a time and pumping the brakes. That forces the fluid and air through the line and out the tube that's open. Eventually you end up with nothing but fluid and can close that line off and do the same for the others. There's a bit more to it, but that's the simplified version.

Bottom line, unlike your engine where the fluid is a lubricant, in your brakes the fluid is a hydraulic fluid. So it's not a catastrophic destruction when it's low, just needs to be properly maintained and you're back on the road.

Hope that helps!

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    side note: after running dry, the master cylinder should be bench bled, and the ABS will need to be bled also. – rpmerf Jan 15 '16 at 21:33
  • The engine oil is a lubricant and a coolant, does the brake fluid act as coolant for the brakes as well? – Max Goodridge Jan 24 '16 at 10:19
  • @MaxGoodridge not really because there is no flow. Engine oil is constantly flowing through the engine so it can carry away heat. The brake fluid is just static. Heat is an issue since a tiny amount of fluid at the caliper gets heated by the brake disk. That's one thing the DOT rating is about. It's how much heat the fluid can handle when the brakes get hot, since boiling the fluid creates air pockets, and again the brakes get spongy or don't work at all. – cdunn Jan 25 '16 at 20:58
  • @cdunn Nice answer, I assume you meant "Heat isn't an issue since..." – Max Goodridge Jan 26 '16 at 6:47

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