I used to have dot 6 at home and it wasn't even silicone based, go figure.
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@J_D 2.0, I meant that having a DOT5 glycol based fluid and then releasing what seems like a minor improvement in DOT5.1 but making it an incompatible silicone brake fluid just leads to confusion and is a stupid choice.
Back then, they should have made the purposeful separation and called it “DOT6“ or better still “DOT6 Silicon” so it’s absolutely obvious its not a backward compatible glycol based fluid, back when DOT6 didn’t exist…DOT5.1 should never have existed…
As is, DOT isn’t the only game in town as there is the ISO standard and vehicle manufacturers can specify their spec own material standards:
Most manufacturers have their own proprietary “material standards.” Some material standards duplicate DOT or ISO standards. In many cases, they are so close that the generic DOT or ISO standard they can be substituted for the proprietary standard. But many material standards are far more stringent than any current DOT standard. Technicians must focus on the manufacturer's material standard, not the generic DOT Standard.
This chart indicates Volvo requires DOT 4, DOT 5.1, and ISO 4925 Class 6 for their newest vehicles. This is not a list of options but an indication that the brake fluid to be installed must meet all three specifications.
I’m not an expert but it seems that the ISO standard is being interpreted as a DOT number which is not strictly correct and may just be sloppy convenient usage but the following ISO class standards exist:
Class 6 Brake Fluid (AKA DOT 6):
ISO 4925 Class 6 (DOT 6) brake fluid has the LOWEST viscosity and is required in many modern automobiles. Class 6 is glycol-based and backwards compatible to DOT 3, DOT 4 & DOT 5.1.
Class 7 Brake Fluid:
ISO 4925 Class 7 (DOT 7) brake fluid has the LOWEST viscosity and the HIGHEST boiling Point. Class 7 matches the boiling point of DOT 5.1 and the low viscosity of Class 6. Class 7 is glycol based and backwards compatible to DOT 3, DOT 4, DOT 5.1 & Class 6.
Oddly there is a DOT4 Class 6 brake fluid so it seems that the two standards DOT & ISO can be combined in odd ways. However, other than slightly different and improved properties, it seems to be the same glycol chemical base as DOT4, just better…
All this wont make much difference to a clutch which just needs good clean fluid but specific properties may be required with modern ABS/ESC systems so it’s worth looking carefully at the manufacturers specs and correctly interpreting them.
"A little dab'll do ya" may work fine for hair care products, but certainly not for brake fluid!
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