Brake Fluid - DOT 3 / DOT 4

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Originally Posted By: kyoo
can I mix amsoil dot3 and dot4
Mix away. DOT 4 has slightly better specs for wet and dry boiling temps but there is no problem mixing. The Valvoline brake fluid I use states it is both DOT 3 and 4. Just don't use DOT 5 unless your vehicle was designed to use it.
 
After so much reading...

DOT4 has better wet and dry boiling points than DOT3. It is also thicker than DOT3, may be a concern at -40, but generally speaking it seems to be better at lubrication for ABS pumps, and it helps seals seal.

DOT4 doesn't absorb water faster than DOT3, it also is less corrosive.

The sticky part about DOT4 is that when it does absorb water, it's temperature performance drops off faster than DOT3, but it still maintains a better wet boiling point.

DOT4+ tends to be less water happy. DOT4-LV is more susceptible to water, and has a recommended change interval of 1-2 years.

DOT5.1 is the best of all worlds, but it's expensive, has a recommended change out of 5 years.

DOT3 should be changed every 1-3 years.
DOT4 should be changed every 2-3 years.
DOT4+ should be changed every 2-4 years.
DOT4-LV should be changed every 1-2 years.
DOT5.1 should be changed every 4-5 years.

Of course there are boosted DOT3 fluids, that can have extra corrosion protection, high dry boiling, extra lubricating additives for ABS, and/or colours.

DOT4 fluids can vary widely as well, lower moisture interaction, boiling temps, viscosities, etc.

Around here the fancy DOT3 fluids cost as much as basic DOT4.

I've switched almost everything over to DOT4, because of the better pedal feel it gives, and the ABS systems seem to work better with it in. Also the temperature capabilities present themselves when dealing with the [censored] under-cooled systems GM likes. Also I've had less corrosion on brake parts since switching over, it's nice not having rear brake cylinder rot and seize.

Of course, the above is just what I remember, and driving conditions will change the change intervals. A water meter to determine how much water is in the fluid, is a very handy tool. No fluid is lifetime, and brake fluid should be checked regularly and flushed when needed. Most shops will bleed a little out when changing pads or the like, but won't flush unless you pay the $100+ to get them to do it. The black goo, coming out of calipers, and the techs give no [censored]. A panic stop or two in short order, and no brakes for you as the pedal hits the floor.
 
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