Brake Fluid Military Testing - DOT 3 v DOT 4 vs DOT 5.1

X15

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Jul 17, 2020
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Very interesting, there seems to be a serious problem with the numbers as presented however...

For instance on page 23 it's stated that "The lowest viscosity observed at -55°C was 4,588 mm 2/s for ISO 6 A", however while Table 12 agrees with this, Figure 19 shows the -55c viscosity for ISO 6 A as 7,755 mm 2/s.

But Figure 20 for ISO 6 B does have a -55c viscosity of 4,588 mm 2/s, it's clear that somewhere the numbers got all swapped around, without a through investigation I wouldn't trust any of them...

5.1 A is stated as being used by the ABS equipped HMMWV, I found the NSN for it (9150-01-694-1600) and it's listed as DOW DBF 565, which is avalible as Wagner DOT 5.1
 
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I just changed all my car DOT 3 fluid with DOT 5.1, so far the brand determines the frequency of change, the DOT rating does not really matter on the water absorption, but brand it is
 

denwood

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Apr 7, 2022
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@X15 , nice find on the 5.1 spec ..that's the information I was hoping for here :)

@kr_bitog, I'd disagree. Water absorption clearly trends higher as you go up in DOT spec. Yes, there are variations, but in general a DOT 3 will absorb less water than a DOT 4, and a DOT 4 less than a DOT 5.1 over the same conditions.
 
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u want the highest wet boiling point which extends changeout interval. presently I use DOT 4 LV which has a high WBP and provides much better brake feel in a wider range of ambient temps. buy brake fluid that is in metal cans since brake fluid can absorb moisture from the air in plastic bottles while sitting on the shelf.
 

X15

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@X15 , nice find on the 5.1 spec ..that's the information I was hoping for here :)

If we assume that Tables 1, 10, and 13 are correct, and adjust the other tables and figures accordingly, then going off the market survey here's my guess for the 5 DOT 5.1 candidates;

5.1 A: We've already established this is DOW DuPont DBF565

5.1 B & C: Both are described in Table 1 as run of the mill DOT 5.1s, the only two remaining fluids that fit the bill for that are the Orthene OSS 13 and MOTUL DOT 5.1

5.1 D: Primarily described as a Low Viscosity DOT 5.1, Orthene OSS 159 is listed as being "DOT 5.1 LV" spec in the market survey results, the only 5.1 so noted, the description in Table 1 is consistent with Orthene's description of their single DOT 5.1 LV on their website

5.1 E: Described as a "Specialty DOT 5.1 said to have improved low temperature viscosity and lubricity performance", this is consistent with Bosch's marketing for ESi6, including the claim of superior lubricity
 
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Apr 21, 2019
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I purchased a new Honda Accord sedan and a wagon back in 1994. The first summer I experienced moderate brake fade in each car with repeated braking on downhill stretches of my commute on 90ºF plus days. Honda OEM was DOT 3, so I switched to DOT 4 and the problem went away for each car for the 15 and 17 years I owned them respectively. I changed fluid every three years or less and never had a failure with brake calipers or ABS. Both cars needed master cylinders after about 10 years due to pedal slowly sinking to the floor. I've been using DOT 5.1 Bosch in my 2007 Accord and 2011 TSX for the past few years. Probably overkill but no issues yet.
 

JHZR2

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I was always at the assumption that DOT 3 absorbed water faster than DOT 4. I had assumed that based on previous Toyota's I've owned that specified DOT3 and a yearly change interval, vs other manufacturers using DOT 4 and specifying a 2yearly change
I was under the opposite belief. DOT4 has a higher dry and wet boiling point, but it absorbs moisture faster than DOT 3.
 
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