Bleed Brakes - Flush Brake Fluid
DOT 3 brake fluid is specified for most vehicles. This brake fluid is hygroscopic which means it will attract and hold water molecules. The water will be absorbed right out of the air and as brake fluid ages it will begin to promote corrosion in many of the hydraulic components found in the braking system. In addition to corrosion, the seals and hoses used in the hydraulic system slowly deteriorate and small rubber particles will become suspended in the brake fluid. Antilock brake hydraulic control valves have very small passages that are easily plugged from the byproducts of corrosion, worn brake hoses and seals. For these reasons most manufacturers recommend replacing brake fluid every two years. I consider it a best practice to replace ALL brake fluid any time you replace front or rear brakes.
DOT 3 brake fluid is the most common however there are other brake fluids used. Most often the correct fluid to use is printed on the cover to the master cylinder reservoir. If you do not find the type of fluid clearly printed on the master cylinder be sure to take the time to look up the proper fluid.
The first step to flushing brake fluid is to make sure all the bleeder screws can be loosened. It is not uncommon to find a rusted shut bleeder screw. Experience will teach you many tricks on how to deal with stuck bleeder screws. It is always important to use a six point socket, or six point box end wrench. If you are unable to remove a bleeder screw you will need to replace that component and the cost for this service should be known before you are half-way through the repair job.
It is very important when flushing or replacing brake fluid to CLEAN the RESERVOIR of the MASTER CYLINDER. Always clean the cover BEFORE removing it. Small particles of dirt will ruin master cylinders and ABS actuators-modulators. Failure to keep hydraulic systems completely clean will always shorten their service life.
Before bleeding brakes it is important to remove old brake fluid from the master cylinder reservoir. This can be easily done with a vacuum brake bleeder. After removing the old/dirty fluid, replace it with fresh/clean fluid. This way you are not pushing dirty fluid on down through the entire hydraulic system. Ignorant mechanics will skip these steps however a professional technician understands how important it is to keep all open hydraulic systems spotlessly clean. Here is the low tech way to remove brake fluid...
During the bleeding or flushing process it is critical to not allow the master cylinder to run out of clean brake fluid. The best way to do this when manual, gravity, or vacuum bleeding is to use a bottle that automatically adds clean fluid to the master cylinder while you are bleeding out the old-dirty fluid. Below is a picture of this tool.
If the master cylinder runs out of fluid the air that enters will need to travel through the entire hydraulic system. With ABS systems this air can get into the hydraulic control solenoids. If this happens you will need to cycle (turn On/Off) these pressure modulator valves. This is done with a scan tool and on some models it can only be done with the manufacturers scan tool. You will have far fewer problems with bleeding brakes if you are careful to not allow the reservoir to run out of fluid. If you allow the master cylinder to run dry you will spend far more time getting rid of that extra air.
Often during brake repair you will need to remove a brake line, wheel cylinder, or caliper. You want to be sure that the brake fluid is not allowed to drain out of the system. This allows air to enter through the top of the system (at the master cylinder) and may be very difficult to bleed back out. One tip is to use a brake pedal depressor to hold the brakes applied.
With the brake pedal held down, air cannot enter from the master cylinder compensating port. With no air entering through the master cylinder, fluid in the top of the system will not drain out. After repairs it will still be necessary to bleed the brakes however the air will be trapped at the lower end of the system near the component that you are replacing.
Your text book explains several methods of bleeding brakes. It is important that you know how to properly perform each method as upon occasion you need to use several methods to successfully get all the air out of the hydraulic brake system.
For all these methods it is important to use the proper bleeding sequence.
We have this book in the shop making it easy to find many important brake specifications. Vehicles with ABS brakes may have very specific procedures that call for cycling the abs control valves using a scan tool. In my experience, if you are careful to not let air get in above these valves, you can often successfully bleed brakes without getting out the scan tool.
Pressure Bleeding can be the fastest most efficient method for flushing and bleeding.
The major drawback is the difficulty in finding the proper adapter to place on the master cylinder reservoir. After hooking up the pressure bleeder ALWAYS watch for any signs of leakage. A poor fit will allow brake fluid to leak all over. This is very bad for the vehicle. Brake fluid will dissolve any paint that it comes in contact with and will deteriorate many plastics. Any brake fluid spills, no matter how small, should be washed off with plenty of water. The water will dissolve away the spilled brake fluid. Brake fluid that is wiped up with a rag will still allow paint, electric insulation, and plastic components to slowly dissolve away. Notice the peeling paint on the pressure gauge shown above.
Once pressure is applied, and you have confirmed that there are no fluid leaks around the master cylinder, go to each bleeder screw following the proper bleeding sequence. Using a clear hose on the bleeder screw can direct the brake fluid to flow into a container so there is no spilling of brake fluid. Allow brake fluid to run out of each bleeder screw until it flows clear - clean brake fluid with no air bubbles.
Pressure bleeding may also require the metering valve to be manually held open. If you only get trickle of brake fluid from the front brake calipers you will need to use a special tool to open the metering valve. If you do not have the tool, or it is too difficult to access the metering valve you must find another way to bleed the brakes.
Vacuum Bleeding is an excellent way to bleed and flush the brake system.
To vacuum bleed you connect the clear hose to a bleeder screw. Connect pressurized shop air to the bleeder bottle. As compressed air blows past the top of the bleeder bottle it creates a vacuum inside the bottle. This pulls brake fluid out of the bleeder screw. Be sure to have the vacuum applied while you are closing each bleeder screw. Also follow the proper bleeding sequence. The only problem I have found is inexperienced technicians will allow the master cylinder to run dry while using the vacuum bleeder. This can cause major problems, especially with ABS systems as it can be very difficult to get all the air back out. Be sure to use the reservoir bottle shown above. When using the vacuum bleeder air will be sucked in past the threads of the bleeder screw. This air does NOT get into the brake lines however you will see that extra air entering the vacuum bleeder bottle. When vacuum bleeding just keep sucking fluid out each bleeder screw until the fluid runs clean. In my opinion Vacuum bleeding is the simplest and cleanest methods for bleeding brakes.
Manual Bleeding sometimes is the only way to get air out of the system. I rarely use this method as it requires 2 people and can cause the master cylinder to fail. The primary disadvantage to this method is having the master cylinder pistons moving past minor corrosion that commonly develops in the ends of the master cylinder. This can damage seals and lead to the eventual failure of the master cylinder. The driver will experience the brake pedal slowly sinking as pressure is kept on the pedal. Replacing the master cylinder is expensive and a hassle. It is not uncommon on older vehicles (over 10 or 15 years old) to have the master cylinder fail soon after manual bleeding has been performed. It is not professional to complete a brake job only to have the vehicle return soon after with a pedal that slowly sinks to the floor due to a bypassing master cylinder. If you perform a manual bleeding sequence be sure the person pressing on the brake pedal does not allow the pedal to sink all the way to the floor!
I recommend that manual bleeding only be used as a last resort. Inexperienced technicians will rapidly pump the brakes, apply too much pedal pressure, and allow the pedal to sink to the floor during this process. Rapid pedal pumping can break up air bubbles and make it more difficult to get all the air out. Allowing the brake pedal to sink to the floor may damage the seals in the master cylinder. The correct method is to only press on the brakes once and hold pressure until the bleeder screw is opened and closed. This will need to be repeated several times for each wheel. The person who is opening the bleeder screw must quickly close it before the pedal forces the piston all the way to the end of the master cylinder.
Gravity Bleeding is very slow however I have found it very useful. To gravity bleed all you need do is remove the cover on the master cylinder and open one bleeder screw. Which bleeder screw you open depends upon the bleeding sequence. Look it up and follow the proper order. The brake fluid should slowly drip out of the bleeder screw. Sometimes you need to add vacuum or gently step on the brake pedal to get the fluid starting to flow out the bleeder screw. Using a clear tube on the bleeder screw can help direct the fluid into a container and avoid any brake fluid spills. Using a clear tube will allow you to see when the brake fluid is running clear. Give each bleeder plenty of time to drip, and keep the master cylinder reservoir full. It may take 15 or 20 minutes per wheel so plan on doing other tasks while using this method of bleeding. Do not open more than one bleeder at a time as it may allow air to be slowly drawn in to one bleeder while dripping fluid out the other. Because it takes so long professionals rarely use this method however gravity bleeding can remove air that other methods miss.
Always properly dispose of the brake fluid. In our shop we drain it into the waste oil container. Always clean and dry any bleeding equipment. The brake fluid left on the tools will damage the paint or plastic of just about everything it comes into contact with. Wash bleeding tools out with plenty of fresh water and completely dry everything before putting them back in their proper location. As you gain experience you will see how much damage brake fluid can do to any painted surface (including the vehicle paint!). Any brake fluid spill must immediately getwashed with plenty of water.