Brake fluid is often forgetten about. There are service items that we all think to check every few months, like oil, filters, brake pads, tires, etc. But as long as the brakes function properly, how often do you think about your brake fluid?

All cars and light/medium duty trucks today have hydraulic brakes, with a fluid carrying force from your foot out to each wheel, and multiplying it greatly. Hydraulic disc brake systems automatically adjust and are very reliable. However, they’re only really good if they’re maintained properly.

Why does brake fluid go bad?

Brake fluid over time will degrade and effect how your car stops. ​The problem with brake fluid is that it’s hygroscopic; it absorbs moisture from the air. The more moisture brake fluid absorbs, the more prone it is to overheating and fading. The older the fluid is, the less effective it will become.

Another problem with moisture is that it corrodes the calipers, wheel cylinders, brake lines, and everything else, working from the inside out. Old, soggy brake fluid can lead to seized calipers or brake lines rusting out from inside.

If you tow, hit the race track, or otherwise use your brakes hard, fluid can get too hot and boil. This can also happen if a caliper hangs up or seizes, rubbing on the disc while you drive  Overheating has a lasting effect on the fluid’s viscosity and effectiveness. If the fluid in the master cylinder smells burned, or is exceptionally dark in color, it’s time to change it.

How often should I change my brake fluid?

Most manufacturers who use a mineral oil based brake fluid typically suggest you flush it and refill with fresh fluid every three years. For some cars, there may not be a factory recommendation, since it will likely be fine for the length of the lease or loan, which the manufacturer calls "lifetime".

But, Haynes still recommends you change your brake fluid every two years, or 30,000 miles. If you drive aggressively, or tow a trailer, it won’t hurt to do it once a year.

Even if the fluid looks okay, you should still change it periodically. You’re not going to see the moisture in the fluid – it happens on a microscopic level.

How can I tell if my brake fluid needs to be changed?

A common misconception is that brake fluid will last forever. That is not the case. Between heat and moisture, it degrades over time and can also cause internal corrosion.

Your brake fluid should always be clear. If it has any darkness or murkiness, or smells burned, it needs to go. In really old cars that have been sitting, you sometimes find the fluid has turned to jelly, which may require not just a flush, but a full rebuild of the master cylinder and calipers.

One man brake bleeding

How to change your brake fluid

Obviously, each car will be different, however, the general procedure remains the same. Before we do, there is one caveat: it is much easier a second person to help you. Yes, there are ‘one-man’ bleeding tricks, but they’re seldom as effective as having a helper pressing on the pedal for you. 

Also, you’ll need to know if your car has antilock brakes or not (pretty much every car made for the last 20+ years has it). If it does, you’ll need to bleed the system as below, but with the engine running in order to do it correctly, because the ABS pump will need to be flushed, too. Check your car’s Haynes Manual for more information, as certain cars require you to start flushing the system at one specific wheel.

The best way to tackle this is with the car up on jack stands, and all four wheels removed before you start. 

  1. Drain the old fluid - Take the cap off the brake master cylinder and, using a turkey baster or something similar, remove as much of the fluid as possible. Put this old fluid in a suitable container for disposal. Once you’ve removed as much as you can, get a lint-free cloth and clean out any debris that might be in the reservoir.
     
  2. Fill it back up - Once that’s done, you need to fill the master cylinder back up with fresh, new fluid, right up to the MAX line. Make sure you use the correct-specification fluid which should be marked on the master cylinder cap. Don’t put the cap back on yet, because you'll need to top it up a few times. You’re looking to push all the old fluid out with the clean, new fluid.
     
  3. On to the calipers/cylinders - Each brake calliper or drum brake wheel cylinder has a bleed nipple on the back. Spray each with some brake clean to clean off the grease and grime, then penetrating oil to free it up.
     
  4. Start with the furthest - For most cars, you start at the corner furthest from the master cylinder. That would be the rear wheel on the passenger side. Affix a small length of rubber hose to the bleed nipple, and then submerge the end of the hose into the container you put the old brake fluid from the master cylinder in. 
     
  5. Time for a helper - With your helper in the driver’s seat, go to the first bleed nipple. Stick a block under the brake pedal to stop it from being pushed to the floor, which is too far.
     
  6. Pump and hold - Have them pump the brakes 3 times, then hold the pedal down. While they hold the pedal, you loosen the bleed nipple and old fluid should quickly come out, along with any air in the line. Close the nipple and signal for your helper to pump the pedal again and hold it. Open and close the nipple again. Keep doing this until new fluid is coming through, without any bubbles.
     
  7. Top ups - Keep an eye on the fluid level in the master cylinder, because you will need to top this up as you progress. If you let the master cylinder reservoir run empty, you will pump air into the lines and have to start over.
     
  8. Rise and repeat - Do the same for each wheel, working closer to the master cylinder as you go.
     
  9. One last thing - When you finish the last caliper, you should have a decent pedal ‘feel’, by which we mean the pedal should feel firm. This is good, because it means the brake system is full of new fluid and not air. Now check the level in the master cylinder one last time, top off if needed, then close it up.
     
  10. Wheels on - With all four wheels off, this is also a great time to rotate the tires. Put the wheels back on, lower the car to the ground, and be on your way.

Remember to always been careful for the first few miles after working on the brakes Check the brake fluid level at the next fuel stop and fill it to the MAX mark if needed.