Hands of car mechanic with wrench in garage. The hand of a car mechanic with a wrench in the workshop.
You may keep a careful watch on the wear of your brake pads. But how about your brake fluid? Did you know brake fluid is not a lifetime fill and should be changed as needed? Unlike motor oil, common DOT 3 and DOT 4 brake fluid is hygroscopic; that is, it draws moisture from the air. It’s why you should use only fresh brake fluid you’ve recently opened, and it’s why the brake fluid in your vehicle may be overdue for a change.
The automotive experts at FDP Virginia Inc., which develops and sells Goodyear Brakes, offer some tips for assessing the health of your vehicle’s braking system, as well as advanced solutions for addressing any issues.
Why Is Moisture in the Fluid a Problem?
Moisture in the brake fluid can cause problems because it lowers the boiling point and can cause corrosion in the braking system. Repairs can get expensive if bad fluid corrodes calipers, brake lines and parts in anti-lock brake system (ABS) controllers. A lower boiling point reduces brake performance in severe braking conditions, such as driving down a mountain road. Although you’ve downshifted into a lower gear, you find you still have to apply a heavy foot to the brake pedal to keep your speed controlled, which heats the brakes and fluid, causing it to boil and form vapor pockets.
Instead of transferring braking pressure to the caliper pistons – pushing the pads against the rotors and braking your vehicle – vapor in the fluid compresses, causing a low, spongy pedal and less braking force. This results in increased stopping distances, or in extreme cases, a complete loss of braking until the fluid cools.
By law, new DOT 3 brake fluid must have a dry (no moisture) boiling point of at least 401 degrees Fahrenheit and a wet (moisture-saturated) boiling point of at least 284 degrees. But just 3% moisture can lower the boiling point to 293 degrees, or just above the bare minimum.
How do you know when it’s time to change it? Your owner’s manual may have a manufacturer’s recommended service interval. If it doesn’t, or if it’s been a few years and the fluid no longer is a clean amber color like vegetable oil, it’s due for a change.
You can also buy test strips online or from a local parts store. Just remove the master cylinder reservoir cap and dip the end of the strip in the fluid for 60 seconds. If you need to locate the cap, check your owner’s manual. Then check the test strip against the colors shown on the package. As the levels of copper increase, the test strip turns more purple. These strips don’t directly check moisture content, but instead check for the presence of copper in the fluid. Copper in the fluid is indicative of corrosion that’s already occurred in the metal brake lines and other brake parts from moisture. And that means the corrosion inhibitors in the fluid are no longer protecting the rest of the system.
How to Get the Old Fluid Out and the New Fluid In
So how do you get the old fluid out and add new fluid? You could pay a shop up to $100 to do it for you, saving you the muss and fuss. Or, if you’re a do-it-yourselfer, it’s a straight-forward operation, especially if the master cylinder is carefully monitored to ensure it never runs out of fluid.
Some vehicles require a scan tool to cycle the pump and valves to bleed air from the ABS controller if air is accidentally introduced as part of the process. It is recommended that you check service information in a shop manual or at another source with vehicle-specific info.
This operation is commonly called a “flush,” although “brake fluid transfusion” may better illustrate the goal of removing as much of the old fluid as possible and replacing it with new. You can save time if you do this when you’re replacing your brakes, as you’ll already have the vehicle on jack stands and the wheels off.
Phone a Friend for This Procedure
Exchanging the brake fluid is essentially the same as bleeding your brakes, which already bleeds out old fluid at the brake caliper (or, for drum brakes, at the wheel cylinder) and requires that new fluid be added to the master cylinder. With a flush, all you’re doing is continuing the process by using more brake fluid to ensure all of the old fluid is removed.
There are “one-man” bleeders and expensive equipment used by repair shops. But for the occasional job for the do-it-yourselfer, it’s easier and quicker to have an assistant in the driver’s seat to work the brake pedal.
How to Bleed
Whether you’ve installed new calipers or are reusing your old ones, the flushing and bleeding procedure is the same. First, clean off the master cylinder cap with a paper or cloth shop towel to prevent dirt from entering. An optional step, especially advised for vehicles with visibly dirty fluid (which, at its worst, can be almost black or rusty,) is to first remove some of the old fluid with a turkey baster. Do not remove more fluid than the minimum fluid amount, which would introduce more air into the master cylinder. Add fluid to the maximum amount and replace the master cylinder cap.
Begin the procedure at the brake caliper or wheel cylinder the farthest from the master cylinder and finish with the brake closest to the master cylinder. For left-hand-drive vehicles (“left” is always as you face forward,) that usually means you’ll begin with the right rear, followed by the left rear, right front, and finish with the left front. Check your vehicle manual or its repair information if you’re unsure.
First, place the box-end wrench on the flats (looks like a nut) of the bleeder screw. The threads may be somewhat corroded, so a six-point wrench is recommended to lessen the chance of rounding off the flats of the screw. Severely corroded screws may even break with sufficient force. Removing a broken-off screw is not practical for most do-it-yourselfers, so replacing both calipers is the practical solution. If present, remove the protective rubber cap from the bleeder screw nipple. Then place one end of the vinyl tubing (3/16” or 1/4”, whichever is a tight fit) over the nipple and the other end in the disposable container. Put an inch or two of fresh fluid in the container so that air is not drawn back into the vinyl tubing as the bleeder screw is closed.
Your assistant should not rapidly pump the brakes, but instead slowly (count to five) depress the brake pedal near the end of its travel and hold it there (not all the way to the floor). While your assistant holds the brake pedal firmly, crack open the bleeder with the hose inserted into the disposable container. Repeat this process as many times as is required until you see only clear amber fluid and no air bubbles from the tubing. You will need good communication with your assistant to coordinate working the brake pedal and the bleeder valves.
Check local regulations, but you can usually dispose of the old brake fluid as part of your household hazardous waste. Many communities offer free annual collections. Torque the wheels to specifications and before taking it out on the road, “driveway-test” the system to ensure the brakes work normally with no mushiness. Now enjoy your newfound firm pedal with the satisfaction that you’ve extended the life of your braking system!
Source: Goodyear Brakes