DOT 3/4 Brake Fluid Tester - NCRS Discussion Boards

DOT 3/4 Brake Fluid Tester

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  • Bob B.
    Very Frequent User
    • March 1, 2003
    • 831

    DOT 3/4 Brake Fluid Tester

    Hi all,

    On a non-Corvette state inspection, the car failed because the brake fluid had too high a moisture content, so I flushed and replaced the brake fluid.

    I got a brake fluid tester and checked my 1960 Corvette master cylinder. It showed high moisture, so I did a flush and replace on that. too.

    Then I realized that these tests are just on the master cylinder, which it seems would be the first place to pick up moisture, so how can that be representative of what is in the lines and wheel cylinders??

    Or what if fluid had recently been added to the master cylinder? That would show low moisture, but would have nothing to do with the rest of the system.

    Would it be better to bleed the farthest brake from the master cylinder and test that?

    Thanks,

    Bob
  • Bill S.
    Very Frequent User
    • April 30, 2002
    • 154

    #2
    Every test I have ever read about or seen says to test at the MC. Somewhere in Chemistry I remember a principle that says if you add 1 drop of liquid to another liquid it will over time evenly disperse itself within the new liquid. So any moisture picked up at the MC would evenly distribute itself throughout the whole system.
    Bill Strobel
    Owner Independent Towing
    Fayetteville, NC
    1979 Corvette White/Red L-82 4 spd
    Only 4,200 miles
    Do It Right or Don't Do It At All

    Comment

    • Mark F.
      Extremely Frequent Poster
      • July 31, 1998
      • 1529

      #3
      Originally posted by Bill Strobel (37873)
      Every test I have ever read about or seen says to test at the MC. Somewhere in Chemistry I remember a principle that says if you add 1 drop of liquid to another liquid it will over time evenly disperse itself within the new liquid. So any moisture picked up at the MC would evenly distribute itself throughout the whole system.
      Here's what Googles AI summary says about it:

      Brake fluids, particularly those based on glycol-ether (DOT 3, 4, and 5.1), are hygroscopic, meaning they readily absorb water from the environment, which can degrade their performance and boiling point.
      Here's a more detailed explanation:
      • Hygroscopic Nature:
        DOT brake fluids are designed to absorb water, which is a common occurrence in brake systems due to seals, microscopic pores, and atmospheric moisture.
      • Water's Impact:
        Water absorption lowers the brake fluid's boiling point, potentially leading to "vapor lock" (where the fluid boils and forms vapor bubbles, preventing proper brake pressure transmission).
      • DOT 5 (Silicone-Based):
        In contrast, DOT 5 brake fluid is not hygroscopic, meaning it does not absorb water, and is used in classic cars that remain in storage for long periods.
      • Importance of Regular Maintenance:
        Due to the hygroscopic nature of DOT fluids, periodic brake fluid replacement is crucial to remove absorbed moisture and maintain brake performance.
      • Water can corrode brake system components
      And the chart below shows how water can degrade fluid performance
      Note the original Y-axis is °C, which I converted to °F on the left-most axis
      DOT 3 is affected the most (bottom red line) - my guess is DOT 5 would be even better than DOT ultra (the top blue line)
      Brake Fluid Moisture vs Boiling Point landscape_001.jpg
      thx,
      Mark

      Comment

      • Duke W.
        Beyond Control Poster
        • January 1, 1993
        • 15676

        #4
        I agree with post #2.

        To do a brake fluid flush start out by siphoning the old fluid out of the MC. Inspect the bottom for any sludge and clean out. Use brake fluid or denatured alcohol, to siphon or sop up with a clean rag. Get it squeaky clean.

        With the MC reservoir(s) clean fill them with fresh brake fluid. If using the two-person pedal pumping method loosely install the MC cover and place clean rags under the MC. The reason for this is that every time the pedal is pushed a little geyser of fluid will shoot up from the compensating port and will make a mess unless the reservoir is covered.

        Start by bleeding the shortest corner from the MC, which would be the left front and left rear on dual circuit systems. Why? Because there is old fluid between the MC and junction block. Why push all this fluid through the longest outlet to expel it. Use the shortest path. Doesn't this make sense?

        I usually do 6-10 pumps at each corner, and go around at least three times. IIRC my SWC's system capacity is 0.65 pint, so flushing a full quart through is three times system capacity, so that should be a good flush. Dual circuit systems have greater fluid capcity, but a quart should be good for most.

        Frequency of flushing is dependent on environmental conditions. A car that sits outside in a damp climate where there is frequent condensing humidity should be done more often than a dry climate where the car is kept inside and rarely sees condensing humidity, and using those test strips to annually check fluid condition is a good idea.

        DOT 3 and 4 fluid is water soluble, so rinsing the rags in water is okay and they can be reused for the next brake fluid flush,

        Duke

        Comment

        • Harry S.
          Extremely Frequent Poster
          • July 31, 2002
          • 5302

          #5
          Originally posted by Bob Baird (39424)
          Hi all,

          On a non-Corvette state inspection, the car failed because the brake fluid had too high a moisture content, so I flushed and replaced the brake fluid.

          I got a brake fluid tester and checked my 1960 Corvette master cylinder. It showed high moisture, so I did a flush and replace on that. too.

          Then I realized that these tests are just on the master cylinder, which it seems would be the first place to pick up moisture, so how can that be representative of what is in the lines and wheel cylinders??

          Or what if fluid had recently been added to the master cylinder? That would show low moisture, but would have nothing to do with the rest of the system.

          Would it be better to bleed the farthest brake from the master cylinder and test that?

          Thanks,

          Bob
          Am I missing something. I've never had my brake fluid checked at a state inspection. Is this something you have in VA to help spend your $$$ with the state?


          Comment

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