The MGA With An Attitude
BRAKE FLUID, Mixing Fluid Types and Water - HT-115A

On 1/23/2017, Gerard Geraci wrote:
"The flushing of my brakes back to DOT 5 went well. However, with all of the dire warnings about mixing DOT 5 and DOT 3/4, I ran a few tests.

I mixed DOT 4 and DOT 5 with water, agitated the mix and let it sit. Within a 30 minutes the DOT 5 floated to the top of the water while the water was absorbed by the DOT 4. After one week, no change (as expected).
DOT 5 and water mix
DOT 5 with water
DOT 4 and water mix
DOT 4 with water
I also mixed some DOT 5 with varying amounts of DOT 4 and agitated the mix.
DOT 4 mixed 20% in DOT 5
DOT 4 20% in DOT 5
DOT 4 mixed 10% in DOT 5
DOT 4 10% in DOT 5
DOT 4 mixed 10% in DOT 5
Top View
DOT 4 10% in DOT 5
All samples have been sitting for nearly 7 days. A few observations.

The DOT 4 and DOT 5 remain liquid, no formation of solids.
The DOT 4 sinks to the bottom of the DOT 5 (since DOT 4 has a higher density).
The DOT 4 absorbs the color from the DOT 5, a curious result.

Just as an additional precaution I will flush the system again with DOT 5 in about a month, as you suggested". -- Gerry

So, that last row of pictures shows what would happen if you flush a DOT 3 or DOT 4 system with DOT 5 fluid without removing all of the DOT 3/4 fluid first. This may not actually harm the seals, but since the fluid types do not mix, the fluid will boil at the temperature rating of the lesser fluid. In short, it will revert to DOT 3 or DOT 4 specification even when most of the fluid is DOT 5. Furthermore, when a small amount of moisture gets into the system it will not mix with DOT 5, but it will be absorbed into the small amount of remaining DOT 4 fluid, which lowers the boiling point even lower.

Best advice is, if you are changing fluid type you should disassemble the whole system for cleaning and blow out all of the old fluid.

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