ABS Brake System Bleeding: Before Bleeding, Check Procedure, Precautions & Bleeding Sequence

The first step to take on any vehicle equipped with ABS that requires hydraulic service is to look up the bleeding sequence, procedure and precautions. This information will give you a starting point. Access of this information can be obtained from one of the consolidated bleeding guides available from various sources, or from one of the electronic service information providers.

This should even be done if only performing wheel-based hydraulic service, such as wheel cylinder or caliper replacement. A good example of why this needs to be done can be seen by looking at what is involved in bleeding the rear brakes on certain vehicles equipped with older Teves II ABS systems or the Delco Moraine III ABS system. The rear brakes on both of these systems can only be bled using the ABS system. Manual or pressure bleeding will not work because of the design of the system. These systems have a built-in pressure bleeder for the rear brakes. The front brakes are bled with a discharged accumulator using either foot or pressure bleeding.

The rear brakes must be bled with a charged accumulator. The process is easy. Simply turn the key on and let the pump/motor charge the accumulator. Once the pump stops, open the right rear bleeder screw and have an assistant depress the brake pedal slightly for about 10 seconds. Repeat the process until all air is removed and then perform the same process on the left rear. Caution must be used not to depress the brake pedal too far because of the high pressures stored in the accumulator.

The rear brakes on these systems are not connected to the master cylinder. Instead, they connect to the boost chamber where power assist is generated. This chamber is charged by accumulator pressure when the brake pedal is applied (See Figure 1). So, unless you are very familiar with the system being serviced, you should get in the practice of looking up what is involved before starting the hydraulic service.

Whether you can perform service will be based on what you find when you look the procedures up and what tools you have available. Let’s start simple and say there are no special procedures listed for the vehicle being serviced, and you are performing only wheel-based hydraulic service. If this is the case, there won’t be much chance of running into trouble if some simple precautions are followed.

The first of these involves those systems that use an accumulator. These systems require "pumping the system down." The procedures for these types of systems will have you pump the brake pedal with the key off anywhere from 10 to 50 times! There is a generic approach to this procedure that will work on all vehicles. With the key off, pump the brake pedal until it is hard. When the brake pedal gets hard. This indicates a discharged accumulator and the system is safe to service.

The next precaution, or step, to take is to make sure you don’t allow the system to run dry during service. This can be done by either capping off the end of the brake line, plugging the brake hose bolt hole or line locking the hose. Notice I said line locking, not vise gripping! Vise grips can damage even a new hose.

The last step in the process when performing only wheel-based service is to use the correct bleeding sequence. There are more than 30 different bleeding sequences in use so you need to make sure you use the right one. Deviating from the OEM bleeding sequence is one way to create a nightmare car.

One of the basic rules to follow when performing hydraulic service on vehicles equipped with ABS is to keep air out of the modulator. This is an important rule to try and adhere to because if air gets inside the modulator, you may not be able to get it out without special tools or procedures. Since most vehicles are equipped with non-integral ABS systems, we will focus on how to prevent air from entering these systems. If you are doing wheel-based only service, the steps given above work fine.

What about if you are doing brake line replacement somewhere after the modulator? You would definitely want to prevent the system from leaking down during the work or you could have big problems on your hands. There is a very simple method to stop the flow of fluid in cases like these. Using a brake pedal depressor, or similar tool, depress the brake pedal about one inch before starting the hydraulic service. Figure 2 shows what is accomplished with the simple process. The fluid path from the reservoir is blocked when the brake pedal is depressed preventing fluid flow through the system. The only precaution is not to take too long repairing the vehicle or the battery may be drained. You can always disconnect the brake light switch at the same time to prevent this from happening.

What if you are performing hydraulic service above the modulator? Should you be concerned about causing problems in these cases?

The answer depends on the system being serviced. If you look it up and there is nothing listed in the comments section, you are pretty safe. Many modulators are straight in, straight out and will not trap air, but others can.

A good case in point is the Chevy and GM trucks equipped with the 4WAL system. The comment section for these vehicles says if replacing the master cylinder on these trucks you must perform the modulator bleeding procedure. If you look this up, you may get intimidated because the procedure is almost two pages long and requires some special tools!

What if I told you there was a way around this that would prevent air from ever entering the modulator and still perform the flush of the system? Well there is. It involves the use of some unconventional bleeding techniques. The bleeding procedures given by the vehicle manufacturers are based on using conventional bleeding techniques. They are assuming air will be passed through the modulator during this process and that is the reason some of them require bleeding of the modulator as part of the process.

Bleeding technology has changed dramatically over the last five years because of the introduction of new tools. These new tools have allowed new techniques to be developed that allow what I call "legal cheats." A legal cheat is a procedure that deviates from the published procedure, but does not affect the quality of the job.

The procedure I am going to outline can be used on any non-integral ABS system. It will prevent air from entering the modulator during master cylinder replacement. It should be noted that the procedure I am outlining references the use of a tool called a Phoenix InjectorTM. The Phoenix InjectorTM is a specialized hand pump capable of performing many different bleeding and flushing techniques. It is possible to perform the procedures using a conventional pressure bleeder or even a vacuum bleeder if you were to make your own adapters and modify the steps slightly.

Remove the master cylinder from the vehicle and discard. This step leaves the primary and secondary brake lines accessible.

The Phoenix InjectorTM is configured as a fluid pump with the suction end of the tool connected to a new fluid source. The output end of the tool is equipped with a discharge hose and bleeder adapter. If you are familiar with the tool, this configuration is the same as for Reverse Fluid Injection (RFITM) bleeding.

The next step involves determining which line feeds the front brakes and which one feeds the rear brakes. On the 4WAL vehicle, it is pretty simple. Look at the reservoir sizes. The secondary section of the master has a smaller reservoir, which indicates it supplies the rear brakes. On FWD vehicles, it is easiest to trace the left front line up to the master to determine the plumbing.

The next step involves connecting the output end of injector to the brake line supplying the rear brakes. Before doing this, the bleeder adapter must be "burped" or air will be introduced into the system. To burp the adapter, place it half way on the brake line and squeeze the trigger a small amount (See Figure 3). Once the adapter fills up with fluid, squeeze it the rest of the way onto the brake line fitting. This setup will allow you to pressure flush the system without the master cylinder. This same process could be performed using a standard tank-style pressure bleeder if you were willing to fabricate the necessary adapters.

After the injector is connected to the rear circuit supply line, connect a capture container to the right rear bleeder screw. Make sure the bleeder is open at least 1/2 turn. I generally like to capture into a gallon jug rather than the bottle supplied with the Phoenix InjectorTM. This eliminates having to constantly empty the capture container during the flushing process. The injector capture hose is equipped with a nifty one-way check valve that prevents any back flow back into the system (See Figure 4).

With the capture container connected, you can now flush the right rear circuit. The 4WAL vehicle is equipped with a front-to-rear hydraulic system. The rear brakes are on one "T" and the fronts on the other. With the capture container connected to the right rear bleeder, we are flushing the base of the "T," and the left branch (See Figure 5). This is the longest part of the hydraulic system and will take the largest volume of fluid. Usually 10 to 12 strokes is all that is necessary to get the new fluid from the injector to the capture container.

Once the right rear is done, close the bleeder and move it to the left rear. The left rear will only take three to four strokes since we are only flushing the remaining part of the "T." Repeat the steps with the right and left front making sure to "burp" the adapter when connecting it to the primary-side brake line.

After the system has been flushed from the lines down, the new master needs to be bench bled. The goal of bench bleeding is to remove all of the air from the four chambers of the master cylinder (See Figure 6). While you could use the stroke method, there are some new techniques available in this area as well. A very effective method that works on most types of master cylinders is to vacuum bleed them. To perform vacuum bleeding, mount the master cylinder in a vise and fill the reservoir with new brake fluid.

Next, install either fittings from your bench bleeding kit, or brake lines into the outlets of the master cylinder. Make sure they are tight enough to form a good seal. Keeping in mind that the goal of bench bleeding is to remove the air from all of the chambers in the master cylinder, we need to do one more step before performing the bleeding process. If we were to apply a vacuum to the secondary outlet, it would only bleed the pressure portion of the secondary circuit (Section "D" of Figure 6).

Changing the fluid path so it fills the no-pressure portion (Section "C" of Figure 6) first is easy. Using a set of small, needle-nose vise grips, depress the piston about 1/4-inch and clamp the vise grips to hold the piston in this position making sure to clamp against the snap ring, not cylinder bore. Depressing the piston covers up the vent port, which changes the fluid path (See Figure 7). Connect the suction hose of the Phoenix InjectorTM, or the vacuum hose of your vacuum bleeder, up to the secondary outlet fitting. Hold the hose in an upright position and apply a vacuum. The vacuum applied to the pressure chamber will cause the primary cup seal to collapse, which fills the no-pressure chamber and pressure chamber. This method only takes a short time to fill both chambers. The process is complete when you see a solid column of fluid. Repeat these steps with the primary outlet.

Now install the master cylinder without attaching it to the vacuum booster. Connect only the brake lines. At this time, the only place there is air in the system is at the line connections. Instead of trying to force this air down to the wheels and have it pass through the modulator, we will push it up through the vent ports in the master cylinder using a unique process called RFITM. Before performing the RFITM process, we must make sure the vent ports are at the highest point in the system. Many master cylinders are mounted at an angle putting the vent ports lower than the rest of the pressure chamber. This is why we did not bolt the master to the vacuum booster. Gently flex the brake lines to position the master so the firewall end of it is higher than the bumper end. This positions the vents at the highest point.

To remove the air at the line fittings, we must perform RFITM on one wheel from each hydraulic circuit. On front-to-rear split systems, I use the RR and RF. On diagonal-split vehicles, I use the RR and LR. The RFI™ process is performed similar to the pressure bleeding of the lines. Open the bleeder at least 1/2 turn and place the bleeder adapter on the bleeder. Remember the bleeder adapter must be "burped" before placing it over the bleeder. Once connected, use one hand to hold the adapter on the bleeder and apply a slow, steady stroke to the trigger. You only have to move the air a short distance, so it should take not more than two full strokes. Repeat this process on the RF wheel and then bolt the master cylinder to the vacuum booster.

When done properly, this procedure will yield the best possible pedal feel and provide a complete base system flush. The entire process, including bench bleeding the master cylinder, can take as little as 30 minutes and only requires one technician.

If it becomes necessary to bleed a modulator, your ability to perform this task needs to be verified before you dig into the job or you could create a big headache for yourself.

Modulator bleeding will generally fall into four categories. The first of these categories is the simplest – modulators that require no special treatment. Figure 8 shows an example of this type of modulator. This is what I call the "block-style" RWAL/RABS valve. When servicing systems with this modulator, you don’t have to have any concerns about getting the air out of the modulator. It’s an example of what I call the "straight in – straight out" modulator.

The next category of modulators are those equipped with a bleeder(s), but require no special tools to bleed. Figures 10 and 11 show examples of two of the more common bleeder-equipped modulators. Figure 10 is the first type RWAL/RABS valve. This modulator is equipped with a bleeder at the dump valve/accumulator end of the valve. This bleeder is there for two reasons. The primary reason is this area of the valve is outside of the normal fluid flow and becomes stagnant. The old fluid in this portion of the valve is one of the primary reasons the dump valve seat fails (Figure 11).

The second reason there is a bleeder at this point is because some of these valves are mounted at a slight angle with this portion of the valve at the highest point. Air in the rear circuit can end up at this point causing pedal problems. To bleed this type, follow the bleeding sequence given in the manual.

Figure 10 shows the Delco VI ABS system. This is another common modulator equipped with bleeders. This modulator is equipped with bleeders at each front solenoid circuit. The solenoid circuits are the highest point in the modulator (Figure 12). These bleeders should be included whether performing a bleed or flush procedure. The front, or left, solenoid circuit is bled first. The right one is bled after the wheels have been bled.

With the simple types out of the way, we move into examples where more involved steps are required. The first of these types was mentioned earlier — the 4WAL system (Figure 13). This modulator is also equipped with two bleeder screws. The only problem is if you open either one and try to bleed it, nothing will come out. These bleeder screws give access to isolated parts of the modulator which require additional steps to bleed.

To complete the fist step in bleeding one of these modulators, you will need two high-pressure accumulator depressors. Install them over each high-pressure accumulator stem as shown on Figure 14. The next step is to allow access to the isolated circuits by loosening the internal bleed screws (Figure 15). There is one for the left and right circuits. The internal bleed screw will allow fluid flow to the left circuit bleeder screw. Bleed the left circuit out until no air is seen and clean fluid is obtained. Close the left internal bleed screw and repeat the process with the right circuit.

The last category is the modulators requiring the use of special tools to allow the bleed process to be performed. The most common of these tools will be a scan tool. Scan tools are often used to open and close solenoids and activate the pump motor assembly. Before diving into these types, you should first make sure your scan tool has this ability with the system being serviced. Many procedures list a manufacturer-specific scan tool or bleeding adapter. Some aftermarket scan tools can duplicate the functions listed, but many cannot. Luckily, the replacement of these modulators is not common place and your chances of running into it are slight. I can’t stress enough about pre-qualifying the job before diving head first into it.

Suppose you have performed hydraulic service on an ABS system that you don’t have the tools to bleed and you think there is air in the modulator? What now? There is a technique that may get you out of trouble in a situation like this. It depends on whether you can get any pedal out of the vehicle at all. Generally, you can get a safe enough pedal to allow the vehicle to be driven under controlled circumstances.

Let me explain what I’m referring to when I say "controlled circumstances." You need to be able to safely drive the vehicle to a parking lot that has a loose friction surface, such as dirt or gravel. If this can be done, there is a good chance you can use the ABS system to pump the trapped air out to the wheels. Once on the loose friction surface, drive the vehicle to a low speed and then engage the ABS system by trying to lock the wheels up, making sure to allow plenty of stopping distance.

Repeat this process several times while trying to make sure all circuits of the ABS have been engaged (usually RF, LF and either rear). Carefully return to the shop and perform the wheel-based bleeding according to the proper sequence. While not foolproof, this method has proven effective for me in the past.

Editor’s Note: Brake & Front End does not endorse any one specific technique or brand of equipment used to bleed ABS brake systems. The bleeding technique and equipment used in the article were chosen by the author to provide the reader with a more detailed and comprehensive look at the bleeding process.