Brake fluid problem
 

I was driving along today and while taking a turn I noticed my brake light on the dash flashed on. I checked the brake fluid level and sure enough it was at the minimum level. I topped it off to between the minimum and maximum but I have a question.
Where did the missing fluid go? Does a car actually use brake fluid?
The car has 195,000 miles on it and it was just a tiny amount of fluid, but isn't it a sealed system? Should I be worried?

Over times you can loose a bit of fluid. Is this still oem fluid? When is the last time you did a brake fluid flush?

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This is normal as your brake pads and rotors wear over time. Now that you've added that bit of fluid to make up for the difference in distance that the pistons have travelled, which really isn't very far, just a few millimeters, multiplied by the total amount of pistons in the system, you'll find that when you replace all of the pads and/or rotors, your level will go back up when the brake job is complete, as long as the system is not opened or bled in the process.

You're fine with what you did, just keep it topped off.

By the way the fluid in these cars should be flushed every 50,000 to 60,000 miles.

As soon as I posted this I figured it out. Yes the pads have worn over time which causes the pistons to extend just a bit. This displaces fluid. Maybe I should check my pads. The last time I replaced them was probably 130,000 miles ago. (They were fine but I replaced them anyway.) Then I bled the brakes and added fluid. Otherwise most of the fluid is what came with the car. Flushing the fluid seems a little drastic. The car has never been off the road so it's not like I'm overheating the brakes.

I would still do a full flush. It s not like you need much to do it. If replacing pads do a flush

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Remember that modern brake fluid is hygroscopic, meaning that under ordinary conditions, it will absorb moisture. That moisture can and will do damage to internal components and steel brake lines over time. Check your owner’s manual to verify what I said.

A simple brake bleed, however is insufficient to flush the entire system. You need to have a dealer-level, bi-directional scanning tool to be able to open and close the ABS valves to completely flush the ABS system along with the lines.

This right here is the reason for flushing the brake fluid. Even though it may look good to you if you were to do a bleed, the fluid that comes out of the caliper would not look very good.

Flushing the brake fluid isn't recommended for off roading...it's recommended because brake fluid is hydroscopic (absorbs water). As it absorbs water it loses the ability to create and maintain the same pressures and is more prone to boiling. Also, water in the braking system causes corrosion to the braking components. It should be flushed every 3-4 years, more if you are in a humid environment. To maintain braking performance and prevent corrosion related failures/issues.

actually, it's 30k miles.

https://www.counterspacegarage.com/b...86-subaru-brz/

Thanks for the correction. I think I confused myself with the 50,000 km value.

Mileage is a rather odd metric for changing brake fluid. I've always used time.

Brake fluid is affected by moisture and temperature.
Moisture is accumulated over time, regardless of miles driven, hence a time based flush is recommended, typically every 2-3 years for road cars, more frequently for track cars (as higher heat tolerant brake fluid tends to be more hygroscopic).
High heat will also damage brake fluid as well as introducing bubbles due to boiling the moisture. That's why track use needs higher temperature fluid and more frequent flushes.

Thanks for all the replies. I was under the impression that the system was sealed. Therefore it could not absorb any water. No more than an unopened bottle of brake fluid sitting on the shelf. But I guess it needs to be vented.
Or does it? If you remove the cap to the master cylinder it appears to be sealed. It seems to have a diaphragm on the inside of the cap. Is this for expansion without a vent?

You will not get air into the system by opening the fill port unless you flush the fluid and let the resivor run dry. Also note that brake fluid that's be opened has a self life of about 1 year as its no longer 100% sealed

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