Bleed and Flush Pointers
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Bleed and Flush Pointers

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As we all know, a bleed and flush is really two separate processes that many times overlap on the same job. Bleeding the system is done to remove all air from the hydraulic system, while flushing is performed to replace the fluid.

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More and more shops are performing a variety of fluid flush services as a method of increasing shop income. Generally, shops purchase a specialized flush machine for each system they want to perform these services on, but this is not always the case with brake fluid flushing. While there are a number of brake fluid flush machines available, it is not necessary to use one to perform a brake system flush.

Shops have three general options when they choose to perform a brake system flush:

Use a dedicated brake system flush machine;

 

Use either a pressure or vacuum bleeder; or

 

Foot bleed the system to flush it.

Note: These are listed in the order of efficiency.

Most flush machines are designed to allow quick setup and semi-unattended operation. These features allow shops to maximize work flow. The same flush can be performed using either a pressure or vacuum bleeder, but generally require the technician to stick with the job.

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Performing the flush using foot bleeding is the least efficient of all methods primarily because it ties up two technicians. Foot bleeding in many cases also results in the lowest quality flush, not because of the method, but because of the time spent performing it.

Regardless whether you are using a flush machine, or pressure or vacuum bleeder, there are some techniques you should be aware of that will enhance the quality of the flush. The quality of the flush and the time necessary to perform it are influenced by the design of the master cylinder reservoir. Most modern vehicles use a screw-cap type reservoir that is usually equipped with a float and associated baffles. This type of reservoir is impossible to drain completely on the vehicle. In most cases, your best effort will result in 50% of the fluid being removed. This is not that bad if the vehicle is only a couple of years old and hasn’t suffered the build-up that can occur in the bottom of these reservoirs. If the vehicle has never been flushed, the 50% of fluid left in the reservoir can be pretty nasty. This fluid and the associated sludge in the bottom of the reservoir can impact the quality of the flush.

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When faced with this scenario, you have two options. The first is to increase the time spent at each wheel in an effort to get most of the old fluid out of the reservoir. At the very least, this will involve putting enough additional fluid through the system to displace the remaining 50% in the reservoir. The second option is to remove the reservoir and clean it off the vehicle. This method allows all of the old fluid to be removed and the majority of the sludge. Generally, a combination of hot water, brake clean and compressed air is used to clean the reservoir.

The second of these techniques involves changing the fluid flow during the flush process. If the brake pedal is left in its normal position during the flush, not all of the old fluid will be removed from inside the master cylinder. Figure 1 helps us understand this. The fluid path during both pressure and vacuum bleeding is from the reservoir, through the vent port into the pressure chamber and then out to the system. Leaving the pedal at rest will leave both no-pressure areas of the master cylinder full of old fluid. Figures 2 and 3 demonstrate this.

Figure 2 shows one circuit of a master cylinder before a flush is performed. The piston is in its normal position. Figure 3 shows the same chambers after a flush is performed. Notice the dark fluid in the no-pressure area. This fluid will mix into the system the first time the brake pedal is depressed.

The fluid path can be easily changed so all areas of the master cylinder are flushed. This is done by installing a brake pedal depressor so the brake pedal is depressed about half an inch (See Figure 4). Depressing the brake pedal moves the primary cup seal past the vent port which changes the fluid path. The new fluid path will be from the reservoir through the replenishing port, into the no-pressure chamber, over the cup seal and then out to the system (See Figure 5). Figure 6 shows the master after flushing when using this method. It is easy to see the difference.

One of the primary reasons shops give customers for the need to perform a brake fluid flush involves the ABS. Customers are often told that the flush will help prevent future internal pr

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The process of bleeding at the wheels until clean fluid is seen does not flush all areas of the ABS modulator. During non-ABS braking, many passages in the modulator are closed off to fluid flow. They only come into play during an ABS stop where pressure modulation is required. It is in these areas where the fluid flush is most important because this is where many of the valves and small passages are found.

In order to flush these areas of the modulator the valves and/or pump have to be cycled. This process is usually performed through the use of a scan tool, or in some cases a dedicated bleed tool. Generally, the system is flushed and then the modulator is cycled to expel the old fluid and bring new fluid into these areas. Once this is done, the system is bled at the wheels again. Not all shops have the equipment necessary to perform this function so it doesn’t get done. If you don’t have the ability to cycle the ABS there is a work around.

Once the bleed and flush is complete, test drive the vehicle to a loose surface parking lot and engage the ABS half a dozen times. This will draw new fluid into those hidden areas and expel the old. In an ideal world, you would take the vehicle back to the shop and bleed at the wheels again to remove the old fluid that was expelled from the modulator.

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There are also modulators that are equipped with bleeder screws and some that require special bleed procedures. Figures 7 and 8 show two such examples. Figure 7 is the Delco VI ABS used on many GM FWD vehicles, while Figure 8 shows the infamous EBC4 or 4WAL. Bleeding the Delco VI is straightforward – you just have to remember there are bleeders on the modulator.

The EBC4 is a different story. Bleeding this system out can be time consuming and requires special steps. Basically, the high pressure accumulators have to be held and the internal bleed screws have to be opened to allow fluid flow to the unit’s external bleeders.

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So what’s the moral of the story? Well it goes like this – just because you get clean fluid out of the bleeders doesn’t mean the system has been properly flushed. In just a few stops and a couple cycles of the ABS system, there can be enough old fluid in the system to cause it to fail one of the industry’s only accepted brake fluid tests. The Strip Dip™ is a test strip that quickly identifies the brake fluid’s ability to inhibit corrosion. The old fluid left in the reservoir, master cylinder no-pressure chambers and/or ABS modulator can be enough to cause a vehicle to fail the test shortly after receiving a flush.

Now that we have discussed the flush end of the process we can move on to bleeding the system. Whenever I talk about air and the hydraulic system I have to include Sid. Sid is the name I have given any air in the hydraulic system. I feel it is necessary to put a name and face to our enemy so I did just that (See Figure 9). Any time we bleed a system out, our primary objective is to make sure Sid is removed. This task is not as easy as it sounds because Sid has some things going for him. The first is that he is sticky. He likes to stick to the various internal parts of a brake system which can increase the difficulty in getting him out. The next thing he has going for him is the fact there are hiding places in some systems that make it extremely difficult to get him out. The last thing he has going for him involves how the bleed process is performed. If too many “shortcuts” are taken, Sid will remain in the system and cause anything from a low pedal to premature pad wear depending on the system and where he is left.

Bleeding the system out is usually performed after performing hydraulic service. There are steps that can be taken during the hydraulic service that can reduce how much of Sid is allowed into the system. Reducing the number of Sid’s in the system will make the bleeding process much easier.

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One of the first things to make sure of is to never let the system run dry. Based on the system’s configuration, allowing this to happen can create some serious problems when trying to bleed the system out. There are a number of methods to prevent this from happening. When changing calipers or wheel cylinders you can simply make the swap quickly so a minimal amount of fluid is lost.

You can also install a line lock to prevent the system from draining. When using line locks it is important to use approved line locks that will not damage the brake hose. When the work involves changing upstream components or lines and hoses there is a simple way to prevent the system from draining. When performing this type of service, install a brake pedal depressor so the brake pedal is applied about 1/2 inch. This covers the vents ports which prevents gravity bleeding.

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As the hydraulic service is performed, the system can be primed by performing gravity bleeding. Gravity bleeding should never be used as a stand alone bleed process due to the fact it will not remove all of the air or old fluid. Generally, gravity bleeding is done as the work is performed. To perform gravity bleeding, drain and clean the reservoir, if possible, and then fill with new fluid. Start the gravity bleed at a wheel that is not being serviced. For example, if the front brakes are being serviced, start the gravity bleed at one of the rears. Keep an eye on the clarity of the fluid and the reservoir level. Switch to the opposite rear wheel once the first is complete. Move to the front as the work progresses. Gravity bleeding will allow the primary bleed process to go quicker because the system has been primed.

Many hydraulic systems will not present any difficulties when performing the bleed process. Sid will be removed simply by following the posted bleed sequence and procedures. Other systems and components will require additional steps to ensure Sid is removed from the system.

Some calipers have a tendency to trap air in and around the pistons. The Ford F250/350 and E250/350 caliper shown in Figure 10 is an example of this type of caliper. If air is suspected in the caliper, and conventional methods do not yield positive results, use one of, or a combination of, the following methods to remove the air:

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  • Unbolt the caliper from its mount and hold the bleeder at the highest point while bleeding.
  • Tap on the caliper as the bleed process is performed. This helps to dislodge any trapped air.
  • Use the brake pedal to pump the caliper pistons out to their maximum extension making sure not to overextend them. This helps “wipe” the air off the surface of the piston.

Certain brake valves have a tendency to trap air. If air is thought to be trapped in a brake valve, many of them can be bled even though they are not equipped with bleeder screws. These valves have to be “burped” to remove the trapped air. Typically, technicians will attempt to bleed these valves by cracking the inlet and outlet lines. This may work on some types of valves, but not on valves similar to the one shown in Figure 11.

This is a combination valve used on certain Chryslers FWD vehicles. The valve is where the diagonal split takes place. Housed inside of the valve are a pressure differential switch and two proportioning valves, one for each rear wheel. On most vehicles, the valve is located either on the firewall to the left of the master cylinder or on the unibody frame rail just under the master cylinder. The design of the proportioning valves allows air to become trapped around the proportioning valve return spring (See Figure 12). This air normally cannot be removed using conventional bleeding methods. If air becomes trapped it will cause a low spongy pedal and possibly a brake imbalance which can result in a side-to-side premature pad wear condition.

The two proportioning valves are held in place by two 3/4″ retaining nuts. The air can be removed using these two nuts.

Start with the proportioning valve that is supplied by the secondary circuit of the master cylinder. Loosen the 3/4″ retaining nut until fluid flow is established.

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With the nut loosened, have an assistant depress the brake pedal. While the pedal is being depressed, tap the valve body with a brass hammer or wooden hammer handle to dislodge any trapped air.

With the pedal held in an applied position tighten the retaining nut. Repeat steps 1 and 2 until no air is seen during the bleeding.

Repeat the above steps with the primary circuit proportioning valve.

Bleed at the wheels using the following manufacturers sequence, in this case – RR, LF, LR, RF.

While the above procedures are specific to the valve shown in Figure 11, they can be applied to valves of similar design.

Air can also get trapped in the master cylinder. If the master cylinder is mounted at an angle, air could be trapped in the high points if the brake line outlets are not at the highest point. The secondary outlet of the master cylinder, shown in Figure 13, is not at the end of the bore. This presents no problem if the master cylinder is mounted level. If this master cylinder is mounted at even the slightest angle, air will be trapped in the end of the bore.

If the vehicle is experiencing a low and/or spongy brake pedal and the master cylinder is mounted at an angle trapped air might be the cause. Use the steps below to check for trapped air.

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Master cylinders mounted at small angles:

Raise the rear of the vehicle to bring the master cylinder to a level position, as shown in Figure 14.

CAUTION: Do not raise the vehicle in an unsafe manner.

If foot bleeding, crack the line fitting on the secondary brake line and have someone depress the brake pedal making sure not to exceed a certain amount of pedal travel. Repeat this process until no air is seen.

Repeat step 2 on the primary brake line. If pedal height and feel are restored, the repair is complete.

Master cylinders mounted at sharp angles:

The steps above cannot be used to check for air in this type of master cylinder. Raising the rear end of the vehicle to a point where the master cylinder is level, would create an unsafe condition. Use the steps below.

Method 1: (use if pressure bleeding)

With the cap adapter and pressure bleeder connected, unbolt the master cylinder from the vacuum booster. Gently position the master cylinder so the secondary end of the master cylinder is slightly lower than the primary side (See Figure 15).

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Connect a capture container to the left front wheel (or the wheel(s) supplied by the secondary outlet). Open the bleeder screw and let fluid flow.

As fluid flows, tap on secondary end of master cylinder to help dislodge trapped air.

Method 2: (use if Phoenix Injector is available)

Unbolt master from vacuum booster. Gently position the master cylinder so the secondary end of the master cylinder is slightly lower than the primary side.

Perform RFI™ (Reverse Fluid Injection) on one wheel in each hydraulic circuit. The RFI will take any trapped air up and out the vent port.

Unconventional Bleed Techniques
When you look up a vehicle’s bleeding sequence and procedure, the information listed is based on using conventional methods. While these methods work for a majority of situations they do not work in all cases. RFI or Reverse Fluid Injection™ was introduced by Phoenix Systems some years ago as a method to remove trapped air. RFI is considered a problem solving process.

This technique involves forcing fluid into the bleeder screws to force the fluid and air up to the master cylinder (See Figure 16). Air naturally wants to travel up so this technique takes advantage of this.

Most people who have never heard of this before are alarmed because fluid is being pushed up stream. They are concerned about damaging the various hydraulic components. There are two points that should be taken into consideration. First, this technique is used only as a secondary bleed technique. The fluid has already been flushed. Secondly, it is done at very low pressures, usually less than 10psi. This prevents forcing any debris upstream. It can use effectively on most vehicles equipped with both ABS and conventional brakes.

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RFI™ relies on the fact that the vent port is the highest point in the system. The fluid path is from the bleeder screw, through the lines, hoses and valves into the pressure chamber of the master cylinder. From there it escapes into the reservoir through the vent port. This is providing the vent port is the highest point in the master cylinder. If the master cylinder is mounted level, the air will escape through the vent port. If it is mounted at an angle, it will be trapped in the end of the master cylinder. To use RFI™ bleeding on this style of vehicle, the master would have to be brought to a level position. Either raise the rear of the vehicle until the master is level, or remove the master cylinder from the vacuum booster and rest it on the booster studs. When raising the vehicle, be careful to do it in a safe manner. Some masters are mounted at too severe an angle to allow the use of this method. On these, remove the master from the booster.

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You might be asking yourself if all this is worthwhile. I think you should first consider that you won’t have to do these extra steps on most vehicles. Next, think back to the last time you or somebody else in the shop spent too much time bleeding a vehicle out. Spending the extra 10 minutes to undo the master cylinder would have been time well spent.

In addition to RFI™, the Phoenix Injector allows the use of another unconventional bleed technique called Pulse Generation. If you have ever tapped on a caliper or other brake component while bleeding, you understand what pulse generation is used for. Air bubbles like to stick to the internal parts of a brake system. Tapping the component is done to dislodge the air bubbles. Pulse Generation™ accomplishes the same thing except in a different way. Bleeders that allow pulse generation have to ability to change the pressure being applied very quickly which creates “pulses” in the system. These pulses break the air free and it is carried out through the bleeder (see Figures 17 to 19). When used in combination with pressure or RFI™ bleeding, it is very effective in getting the air out. The pulses generated impact the entire hydraulic circuit being bled, not just the component.

The last method I want to discuss involves air trapped in ABS modulators. As I said before, some modulators require special tools to allow bleeding. So what do you do if these tools are not available and you suspect air has been trapped in the modulator?

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The best method to use involves performing the best possible bleed at the wheels to restore the pedal height as much as possible. If the pedal can be restored to a point where the vehicle can be driven safely, take it to a loose surface parking lot and make 6 to 12 hard stops making sure to engage the ABS. As the ABS cycles, it will pump any air out into the system. Return to the shop and rebleed at the wheels. Again, I want to stress do not drive a vehicle with an unsafe pedal.

There is one other technique I want to include before I wrap this article up. One of the problems encountered when selling hydraulic service in the rust belt areas is the inability to open all bleeders. For example, you sell a complete front brake job on an older FWD and when you check the rear bleeder screws, there is no way they are going to open. One option would be to sell wheel cylinders, but this is not always practical. You might say: “So what, I’ll just line lock the front hoses when I swap the calipers and all will be OK.”

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This is not always the case because once you remove the line locks, air in the bottom of the hose and caliper move up into the system. It is true that on many vehicles the air will come right back down when bleeding, but sometimes Sid finds a hiding spot, or worse yet, ends up at the opposite wheel in that circuit. To prevent this, simply leave the line lock on until you are ready to bleed that wheel. If foot bleeding, apply pressure to the pedal, open the bleeder and then remove the line lock. This forces any air down and out. The same is true for pressure bleeding. Pressurize the system before removing the line lock.

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Generally, using some combination of the methods described in this article will allow you to restore virtually any brake pedal in a reasonable amount of time. Knowing which to use, and in what combination, comes with experience.

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