Preventing Air from Entering ABS Modulator During Service

Preventing Air from Entering ABS Modulator During Service

Problem: Some ABS modulators require special tools and procedures to remove trapped air and many manufacturers require modulator bleeding as part of master cylinder service.

Cause: Manufacturers assume master cylinder replacement will result in air entering the ABS modulator. Modulator bleeding is added to the steps of master cylinder replacement to prevent air from remaining in modulator.

NOTE: Many shops don't have all the tools necessary to bleed the large variety of ABS systems in production. Preventing air from entering the modulator is a far easier method of dealing with this issue.

Solution: All current bleeding methods included in service manuals are based on conventional bleeding techniques. Unconventional bleed techniques allow many systems to be bleed without using the special tools or procedures. When replacing a master cylinder on vehicles equipped with non-integral ABS systems use the following steps:

1. The first step is to flush the system from the lines to the wheels. This is done after removing the master cylinder by pressure bleeding directly through the brake lines. Just make sure to "burp" the adapter (See Figure 78.1) before putting it on the brake line. Once connected hook your capture container up to the furthest wheel on that circuit. Apply pressure and bleed until clear fluid is seen.

screen-shot-2016-09-26-at-9-53-49-am Figure 78.1

2. Repeat on other wheel on that circuit. Move adapter to other line and repeat. This will provide clean fluid and no air from the brake lines to the wheels.

NOTE: On 3 & 4 outlet master cylinders use this procedure on each brake line.

3. Next bench bleed the master cylinder and install the lines to the master cylinder without bolting the master cylinder to the booster. At this point the only place there should be air is where the lines meet the master. Which is easier, to push the air to the wheels or to push it up through the vent port? If we push it through the vent port it will be faster and we keep the air away from the modulator.

4. To complete the process we would Reverse Fluid Inject (RFI) one wheel one each hydraulic circuit (See for more information on RFI bleeding) (See Figure 78.2). On a front to rear split system I use the RR and RF, on a diagonal split system I use the RR and LR. This technique is not only good for ABS vehicles but will work on any vehicle where the master cylinder is being replaced. When done properly it yields a fantastic pedal.

screen-shot-2016-09-26-at-10-03-25-am Figure 78.2

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