Brake fluid is an integral part of the braking system, but often overlooked unless there is a problem — in which case it could already be too late. Knowing when and how to check brake fluid can keep you safe. Manufacturers vary wildly in their recommendations for how often the fluid should be changed, ranging from every 15,000 miles to those who strive to make brake systems that never need a single flush. Consult your owner’s manual for the proper fluid change intervals (and the properly rated DOT brake fluid to replace it with), and every six months or so, give it a quick visual inspection just to make sure things are as they should be. Here’s a primer on what you should be looking for:
To check the level and condition of your brake fluid, you’ll need to locate the reservoir under the hood, which on most modern vehicles sits atop the master cylinder, directly behind the firewall on the driver’s side. The majority of reservoirs are clearly marked with min and max fill lines and are translucent so you can see the fluid level without opening the cap. When in doubt consult your owner’s manual for the exact brake fluid reservoir for your particular vehicle.
Generally speaking, your level should not fall below the minimum line unless you have a problem. Your vehicle’s hydraulic brakes are a closed system so there is never a situation where is it acceptable for a vehicle to “use” brake fluid and need to be topped up. Most likely, low levels indicate a need for new brake pads and/or shoes. Over time as the brake pads and shoes wear, the brake calipers and wheel cylinders will retain more fluid in their cylinder chambers. This is not a life-threatening situation, but it’s time to have a skilled mechanic evaluate the condition of your brakes to see if they need replacing. However, a level that has dropped very quickly could indicate a leak in the system and requires immediate attention. Topping off your fluid in either of these circumstances will only be a very temporary solution.
New brake fluid should be clear, and light amber in color. If a quick visual inspection reveals dark, rusty or even inky fluid, it is time for a change. One characteristic of brake fluid is that it attracts and absorbs moisture from its environment over time. When water gets into the brake system, two things can happen.
First, it can rust the metal components in the system, causing early wear as bits flake off into and contaminate the fluid or clog the lines. Second, it lowers the boiling temperature of the brake fluid itself, so that long downhill rides or sudden stops might be too much for your brakes to handle. Moisture in brake fluid has a drastic effect on the fluid boiling point, reducing it by well over 100 degrees or more. If the moisture level in the brake fluid is too high at the same time the brakes are under heavy use (and therefore high temperatures) the fluid can actually boil. If the fluids boils you are no longer trying to stop with a non-compressible fluid but with a compressible gas. Pushing the brake pedal with boiling fluid will likely end up with the pedal on the floor and and a loss of braking ability!
Many times brake fluid problems are apparent upon visual inspection of the fluid, but you can also invest in tools such as chemical test strips to check the amount of copper in the fluid or an optical refractometer to check for moisture. Brake fluid test trips are inexpensive and easy to use. Simply open the brake master cylinder and dip a test strip into the brake fluid. Then compare the results of the test strip to the chart on the brake test strip package. You are testing for the amount of copper in the brake fluid. A copper level of 200 parts per million (ppm) or higher means it is time for a brake fluid flush. There are also digital brake fluid testers that can measure the moisture content of brake fluid. These only require dipping the probe of the tool into the fluid for an instant moisture content reading. A brake fluid moisture reading of more than 3% means time for a system flush and new fluid.
Fluid is the ideal medium for a braking system because it’s incompressible and therefore able to deliver force directly from your foot to the wheels. However, liquids have their pitfalls, namely, their propensity to boil. If, as mentioned earlier, water has breached the system or too much heat builds up, boiling can actually produce gas bubbles that are compressible and will cause the pedal to become spongy — or worse, to fail altogether. Burned brake fluid needs to be replaced regardless of suggested change intervals.
Knowing how to check brake fluid is a handy and an easy enough skill to master, and knowing when to change it can help keep you safe on the road. If there is ever any doubt as to the condition of your brake fluid simply go ahead and replace it. Brake fluid is inexpensive and you can be confident knowing it has been changed. Just remember to use new brake fluid (or fluid that was open less than 12 months). Brake fluid can absorb water from the air even while sitting on the shelf if the bottle has been opened. Old brake fluid should be recycled.
Check out all the chemical products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to check brake fluid, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
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Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter. In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.