Brake Fluid Service and Bleeding

Brake fluid carries the driver's pedal force to every wheel. Liquid does not compress, so a healthy system feels firm and direct. Air does compress, and water lowers the fluid's boiling point. Both can leave a driver with a soft pedal, or no pedal, when it counts. Bleeding removes air, and a fluid exchange replaces old fluid. They use the same skills.

What You Will Be Able to Do

• Identify the correct fluid and keep incompatible types apart.
• Explain why brake fluid ages, and judge when it needs replacing.
• Choose a bleeding method.
• Bleed a system in the correct order without running the reservoir dry.
• Tell when a scan tool is needed for ABS bleeding.

Brake Fluid Types

The fluid type is usually printed on the master cylinder cap and listed in the owner's manual. DOT in the name refers to the US Department of Transportation fluid standard. The boiling points below are the minimums in that standard. "Dry" means new fluid, and "wet" means fluid that has absorbed water, which is closer to real use.

Most modern passenger vehicles use DOT 3 or DOT 4. Use exactly what the vehicle maker specifies. Some newer vehicles call for a low-viscosity DOT 4 that suits their ABS and stability systems.

DOT 5 is the odd one out. It is silicone-based, and it is not compatible with most ABS systems or with glycol fluids. Do not confuse DOT 5 with DOT 5.1, which is a glycol fluid. Never mix DOT 5 with DOT 3, DOT 4 or DOT 5.1. It does not blend with them, and it can damage seals and valves in a system that was not designed for it.

Why Brake Fluid Goes Bad

Glycol brake fluid is hygroscopic, which means it absorbs water from the air. Water gets in through hoses and seals and whenever the cap comes off. This causes two problems.

1. Lower boiling point. Water boils at a lower temperature, and hard braking heats the fluid at the caliper. Boiling creates vapor bubbles, which compress, so the pedal goes soft and braking fades.
2. Corrosion. Water attacks steel lines, caliper bores and ABS valves from the inside.

Fresh fluid is usually clear to light amber. Dark brown fluid is old and contaminated, and its boiling point is reduced.

Color alone is not proof, so use a test.

Many shops replace fluid when it tests at about 3 percent water. The usual time guideline is every 2 to 3 years, whatever the mileage. Some vehicle makers use a different interval, and the maker's schedule wins. Replace it too if the system has been opened for major repairs or if the fluid is visibly contaminated.

Handling Brake Fluid Safely

• Wear safety glasses and nitrile gloves. Flush any eye contact with water for at least 15 minutes.
• Brake fluid strips paint. Cover the fender and wash off any drips with water right away.
• Use fluid from a sealed container and close the cap as soon as you finish. An opened bottle keeps absorbing water from the air.
• Never reuse fluid that has been drained or bled.
• Keep petroleum products out of the system. A small amount of oil, power steering fluid or engine oil can swell every rubber seal. If this happens, the whole system may have to be flushed and its rubber parts replaced.
• Collect used fluid in its own marked container. Do not pour it down a drain, and do not add it to the used oil tank unless your recycler allows it.

When You Need to Bleed

Air gets into the system any time you open a line, replace a hose, caliper or master cylinder, or let the reservoir run low. The classic sign is a soft or spongy pedal. If the soft pedal returns after a good bleed, look for a leak or a failing master cylinder.

Bleeding one corner may be enough after a single caliper or hose job, if the reservoir never ran low and the service information allows it. Bleed the whole system if the master cylinder or ABS unit was involved, or if the pedal is still soft.

Bleeding Order

The general rule is to start at the wheel farthest from the master cylinder and work toward the closest. On a left-hand-drive vehicle, that gives:

1. Right rear (RR)
2. Left rear (LR)
3. Right front (RF)
4. Left front (LF)

Some systems use a different order. Diagonal split systems may use RR, LF, LR, RF, and many ABS systems have an order of their own. The service information for that vehicle always wins.

Bleeding Methods

Pressure bleeders typically run at about 10 to 15 psi. Use the equipment maker's and the vehicle maker's figures, and make sure the tank holds the correct fluid type.

Manual Bleed Step by Step

Raise the vehicle, rest it on the locks and remove the wheels if you need access.

1. Prepare the bleeders. Find all of them. Clean them, and free up any that are stuck with penetrating oil and patience. A snapped bleeder turns a simple service into a caliper job.
2. Fill the reservoir. Fill it to the MAX line with fresh fluid.
3. Set up the first wheel. Place a box-end wrench on the bleeder, then attach a clear hose that runs down into a catch bottle containing a little clean fluid. The end of the hose should stay under the fluid.
4. Press the pedal. Your helper presses the pedal slowly, then holds it down and calls out "holding".
5. Open the bleeder about a quarter turn. Fluid and air flow out, and the pedal sinks.
6. Close the bleeder before the helper lets up. If it is left open, air can be drawn back in.
7. Release. The helper lets the pedal up slowly.
8. Repeat until the fluid is clear and has no bubbles.
9. Check the reservoir often, every few strokes, and top it up. Move to the next wheel in the sequence.
10. Tighten each bleeder to spec. A typical bleeder is small and easy to snap, so do not overdo it. Refit its dust cap.
11. Check the pedal. With the engine off, it should be high and firm. With the engine on, it should drop slightly and then stay firm.
12. Check for leaks at every bleeder, then wipe the area clean.

Warning: Never let the reservoir run dry. If the level falls too far, air is drawn into the master cylinder and the lines, and you may have to bleed the whole system again, sometimes with a scan tool.

Before the vehicle leaves, road test it at low speed first. Confirm the pedal stays firm and that the brake and ABS lights are off.

Full Fluid Exchange

An exchange is simply a bleed that goes on until the new fluid reaches each wheel.

1. Draw most of the old fluid from the reservoir with a clean syringe. Do not empty it fully.
2. Refill with fresh fluid.
3. Bleed each wheel in sequence until the fluid coming out changes from dark to clean. Do not stop at the first clear drop. Watch the color in the clear hose.
4. Keep the reservoir topped up throughout.

Worked Example: How Much Fluid

A typical car system needs about a quart of fluid for a full exchange, and larger trucks can need more. If you start with a 12-ounce bottle on a full exchange, you will run out partway and risk drawing air. Open a fresh 32-ounce container or two 12-ounce bottles before you start. Close each one once you have used what you need.

New Master Cylinders

Bench bleed a new master cylinder before you fit it. Clamp it in a vise, fit the bleed tubes that come with it so that they feed back into the reservoir, fill the reservoir, and stroke the piston slowly until no bubbles appear. Installing a dry master cylinder can trap air that may never fully bleed out on the car.

ABS Bleeding

Standard bleeding works for most routine jobs on ABS vehicles, as long as air has not entered the ABS unit itself. Bleeding with a scan tool becomes necessary when air may have reached the valves.

A bidirectional scan tool runs the ABS pump and cycles the valves to move trapped fluid and air, usually in stages between manual bleeds.

• Normally mandatory: When the ABS module or hydraulic unit has been removed or replaced, unless it came prefilled and the procedure says otherwise.
• Often needed: When the reservoir ran dry, the master cylinder was replaced, or a soft pedal stays after a normal bleed.

Pushing a caliper piston back without opening the bleeder sends dirty fluid back through the ABS unit, where debris can harm the small solenoid valves. Opening the bleeder during retraction lets that fluid out at the caliper instead.

Common Problems

Common Mistakes

Check Your Understanding

1. A customer's reservoir cap says DOT 3, and all you have is DOT 5. What do you do? Do not use it. DOT 5 is silicone and is not compatible with that system. Get the correct fluid.
2. The fluid looks only slightly dark, but it is four years old. What do you recommend? Test it, and recommend replacing it. Age and moisture matter more than looks. Check the maker's schedule.
3. Partway through a bleed, you find the reservoir is nearly empty. Why is that a problem, and what should you do? Air may have been drawn in. Fill the reservoir, then rebleed from the start and check the pedal.
4. You replaced an ABS hydraulic unit, and the pedal is soft after a manual bleed. What is the next step? Do the scan tool bleed procedure from the service information.

Key Takeaways

• Use the fluid the maker specifies. Keep DOT 5 and petroleum products out of glycol systems.
• Glycol fluid absorbs water, which lowers its boiling point. Replace it about every 2 to 3 years, or as the maker says.
• Bleed from the farthest wheel to the nearest, usually RR, LR, RF, LF, unless the service information differs.
• Never let the reservoir run dry.
• ABS module work usually calls for a scan tool bleed.
• Keep used fluid separate, and wipe spills off paint right away.