Bleeding brakes can be an extremely frustrating job. There seems to be a bit of black magic involved with the bleeding process. Sometimes it will work perfectly, and then other times it seems like you end up with a lot of air in the brake system. The best strategy to follow when bleeding your brakes is to repeat the procedure several times in order to make sure that you have removed all the trapped air from the system.
There are currently two popular methods of bleeding the brake system: pressure bleeding and vacuum bleeding. Pressure bleeding uses a reservoir of brake fluid that has a positive air pressure force placed on the opposite side of the fluid, which forces it into the brake system. Vacuum bleeding is where you fill the reservoir, and then apply a vacuum at the bleeder screw to pull fluid through the system. In this article we will focus on pressure bleeding. In my experience, I've found that 7-12 psi works just fine to pump all the old fluid out.
One of our favorite tools for pressure bleeding is the Motive Products Bleeder. The system has a hand pump that you can use to pressurize the brake fluid up to 30psi. A small gauge on the front of the brake fluid reservoir indicates the pressure of the brake fluid inside. The very large reservoir can hold about two quarts of brake fluid: more than enough for most brake flushing and bleeding jobs. Retailing for about $50 online from PelicanParts.com, the bleeder kit is a very useful and cost-effective tool to have in your collection.
The system bleeds by pressurizing a bottle filled with brake fluid from air from an internal hand pump. The procedure is to add fluid, attach the bleeder to the top of the reservoir cap, and pump up the bleeder bottle using the hand pump. This will pressurize the system. Check to make sure that there are no leaks around the bleeder, or where it attaches to the top of the master cylinder reservoir. Make sure that the seal on the inside of the reservoir cap is seated correctly as well.
Now start bleeding the system. Start by jacking up the car and placing it on jack stands. (See our article on Jacking up your Mk4 Jetta for more info). Remove all four wheels and start at the right rear caliper. Starting at the furthest caliper from the master cylinder helps to get most of the air out of the system.
Carefully pry the rubber caps off the bleeder screws and attach a clear plastic hose to the outside bleed screw, place the other end in a jar, and then open the valve by turning the bleeder screw counter-clockwise with a 11mm wrench. Watch the fluid in the clear line for air bubbles. Keep the bleeder valve open until all air bubbles have disappeared. Then close the valve. Once all the air is bled, move to the next caliper and repeat the process. Bleed them in this order: right rear caliper, left rear caliper, right front caliper, left front caliper.
Make sure that you don't run out of brake fluid in your reservoir, or you will have to start over again. It is wise to start out with about a 1/2-gallon of brake fluid in the pressure bleeder, and another 1/2-gallon on the shelf in reserve. Depending upon your car, and the mistakes you may make, I recommend having an ample supply. When you are finished using the power bleeder, be sure to empty out the brake fluid and store it in a sealed container if you intend on using it in the future.
As you are removing fluid from the calipers, it will be emptying the bleeder reservoir. If the reservoir goes empty, you won't be able to bleed the system, as you will have introduced air into the ABS pump. In this instance, you'll need access to diagnostic software to trigger the ABS pump into what is called 'bleed mode'. This mode cycles the pump to purge the air out.
Another important note here is that when the system is pressurized, you want to slowly open the pressure bleeder reservoir first. This will relieve the pressure on the system and will draw fluid back into the reservoir. Do not unscrew the cap at the master cylinder reservoir with the system pressurized. This will spray brake fluid all over the inside of your engine compartment.
If you are installing a new master cylinder, it's probably a wise idea to perform what is called a dry-bleed on the workbench. This is simply the process of getting the master cylinder full of brake fluid and 'wet.' Simply add some brake fluid to both chambers of the master cylinder, and pump it a few times. This will save you a few moments when bleeding the brakes as it eliminates any air pockets that will be inside the cylinder.
Make sure that all the bleeder valves are closed tightly. Disconnect the pressure system from the reservoir. Get your helper to press down repeatedly on the brake pedal at least five times, and then hold it down. Then open the bleeder valve on the right rear caliper. The system should lose pressure, and the pedal should sink to the floor. When the fluid stops coming out of the bleeder valve, close the valve. Tell your helper to let their foot off of the pedal. Do not let them take their foot off until you have completely closed the valve. Repeat this entire procedure for all the valves in the same order as described previously.
I recommend that you use this procedure as a final step, even if you are pressure bleeding. The high force associated with the pressure from the brake pedal can help free air and debris in the lines. If the brake fluid doesn't exit the screw quickly, then you might have a clog in your lines. Brake fluid that simply oozes out of the lines slowly is a clear indication that your rubber lines might be clogged and constricted. Don't ignore these warning signs: check out the brake lines while you are working in this area (See our project on replacing brake lines).
Let the car sit for about 10 minutes. Repeat the bleeding process at each corner. The pedal should now feel pretty stiff. If the pedal still feels spongy, make sure that you have the proper adjustment on your rear calipers or drum shoes. Also, you may need a new master cylinder, have a leaky caliper, or have old spongy flexible brake lines.
If you find that your ABS equipped car feels spongy on the brake pedal, take the car to a deserted parking lot and engage the ABS system by slamming the brakes a few times. Then go back and re-bleed the system: it may take care of the spongy pedal, although this not a factory recommended procedure and it may not eliminate all the air from the system.
Another important thing to remember is that brake fluid kills paint. Brake fluid spilled on paint will permanently mar the surface, so be very careful not to touch the car if you have it on your hands and clothing. This of course, is easier said than done - don't bleed the system in a tight garage. The probability of spilling on yourself and then leaning against your car is too great. Rubber gloves help to protect yourself from getting it on your hands and your paint. If you do get a spot on your paint, make sure that you blot it with a paper towel - don't wipe or smear it. It's also important not to try to clean it off with any chemical or other cleaning solutions.
Remove the protective cap from the brake fluid reservoir and attach the connection for the pressure bleeder. Shown here is the Motive Products Power Bleeder. Available for about $50 from PelicanParts.com, it is a huge time-saver when it comes to bleeding your brakes.
Front and Rear Calipers: Remove the rubber cap off the bleed nipple and attach a section of clear plastic tubing. Open the 11mm bleed nipple in the caliper by about a quarter of a turn. Let the brake fluid run out of the caliper until no more bubbles appear. Close the bleeder valve and put the protective cap back on. Repeat the process for all calipers starting from the furthest caliper from the reservoir.