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Problem: Leaving old fluid in master cylinder no-pressure chambers during pressure or vacuum bleeding

Cause: When performing pressure or vacuum bleeding the fluid flow created by the bleeding process leaves the fluid in the master cylinder’s no-pressure areas virtually untouched. This old fluid will mix with the new fluid once the brake pedal is depressed.

The reason this occurs is because the fluid path during pressure and vacuum bleeding is (se Figure 75.1)

  • From the fluid reservoir
  • Through the vent port
  • Into the pressure chamber
  • And out into the system to the bleeder

Screen Shot 2016-09-02 at 2.29.06 PM Figure 75.1

The end result looks something like what is shown in Figure 75.2. The old fluid in the no-pressure area of the master cylinder is very evident in this see through master cylinder. The old fluid can result in the fluid looking “dirty” shortly after the flush was performed and can result in the fluid failing some of the brake fluid tests available.

Screen Shot 2016-09-02 at 2.32.16 PM Figure 75.2

Solution: To flush the no-pressure areas of the master cylinder follow the steps below:

1. Remove the old fluid from the reservoir and fill with new fluid of the correct type.

2. Use a brake pedal depressor to depress and hold the brake pedal in a slightly applied position. Usually 1/2″ to 1″ is enough. This step positions the master cylinder primary cup seals past the vent ports as shown in Figure 75.3. This step changes the fluid path. The fluid path is now

  • From the fluid reservoir
  • Through the replenishing port
  • Into the no-pressure chamber
  • Over the primary cup seal
  • Into the high pressure chamber
  • And out into the system to the bleeder

Screen Shot 2016-09-02 at 2.36.14 PM Figure 75.3

3. Perform the pressure or vacuum bleeding using the proper sequence.

4. Remove the brake pedal depressor when complete.

Figure 75.4 shows the result when these steps are performed. The difference between 75.2 and 75.4 is obvious. Taking this simple extra step can help ensure a quality flush.

Screen Shot 2016-09-02 at 2.40.26 PM Figure 75.4

Phoenix Systems Recommendation: Along with the suggestions in this article, we also recommend to look into reverse brake bleeding. Phoenix Systems has patented classic reverse brake bleeders that simplify the brake bleeding process. Air naturally wants to rise, and the reverse bleeders take advantage of this process and push the air up and out the master cylinder. Follow the link below to find out more.

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