What a Brake Fluid Flush Actually Does

Why does sealed brake fluid absorb water?

Most brake fluid is hygroscopic, which means it attracts and absorbs moisture. The system is closed, but rubber hoses and seals are slightly permeable and the reservoir vents to the air, so small amounts of moisture find their way in over time.

This happens whether you drive a lot or a little. A car that sits for long periods still has fluid that ages, which is why the replacement interval in your manual depends on time as well as use.

What does moisture do to the pedal and the components?

Water in the fluid lowers its boiling point. Under hard or repeated braking, heat from the brakes can raise the fluid temperature enough to form vapor bubbles, and vapor compresses in a way liquid does not. The result can be a soft pedal that sinks when you need it.

Moisture also promotes corrosion inside calipers, brake lines and ABS parts. Over time, that can lead to sticking pistons or damaged components that are more expensive to repair than the fluid service would have been.

Flush vs. bleed vs. top-off: what is the difference?

• Flush: old fluid is pushed out of the system and replaced with new fluid, so the whole circuit is refreshed.
• Bleed: a small amount of fluid is released at each wheel to remove trapped air, usually after a repair that opened the system.
• Top-off: fluid is added to the reservoir to bring the level up. It does not remove old fluid or air.

Be careful with the last one. Brake fluid level drops slowly as pads wear, but a level that falls noticeably can also mean a leak. Topping off and driving on can hide a problem that needs to be found. If the level is low, have the system checked.

Where do I find my interval, and how does the shop test the fluid?

Your owner's manual lists the manufacturer's recommended brake fluid interval and the correct fluid type for your vehicle. Follow that rather than a number you hear elsewhere, since requirements vary between makes.

During a brake inspection, a technician can check the fluid level and condition along with the pads, rotors and calipers. If the fluid looks questionable, ask what testing they use and what the quote includes before you approve the work, and ask about the current special when you book.