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DOT3

DOT3 brake fluid is the "conventional" brake fluid used in most vehicles.

Advantages:

DOT3 fluid is inexpensive, and available at most gas stations, department
stores, and any auto parts store.

Disadvantages:

DOT3 fluid eats paint!

DOT3 fluid absorbs water very readily. (This is often referred to as being
hydroscopic.) As such, once a

container of DOT3 has been opened, it should not be stored for periods
much longer than a week before use.

Since DOT3 fluid absorbs water, any moisture absorbed by the fluid can
encourage corrosion in the brake lines

and cylinders.

DOT4

DOT4 brake fluid is the brake fluid suggested for use in some late model
cars.

Advantages:

DOT4 fluid is available at most auto parts stores, and at some (but not
all) gas stations or department stores.

DOT4 fluid does not absorb water as readily as DOT3 fluid.

DOT4 fluid has a higher boiling point than DOT3 fluid, making it more suitable
for high performance applications

where the brake systems are expected to get hot.

Disadvantages:

DOT4 fluid eats paint! Small leaks around the master cylinder will eventually
dissolve away the paint on your

bodywork in the general vicinity of the leak, and then give rust a chance
to attack the body of your car!

DOT4 fluid is generally about 50% more expensive than DOT3 fluid.

Since DOT4 fluid still absorbs some water, any moisture absorbed by the
fluid can encourage corrosion in the

brake lines and cylinders.

DOT5

DOT5 brake fluid is also known as "silicone" brake fluid.

Advantages:

DOT5 doesn't eat paint.

DOT5 does not absorb water and may be useful where water absorption is
a problem.

DOT5 is compatible with all rubber formulations. (See more on this under
disadvantages, below.)

Disadvantages:

DOT5 does NOT mix with DOT3, DOT4 or DOT5.1. Most reported problems with
DOT5 are probably due to some

degree of mixing with other fluid types. The best way to convert to DOT5
is to totally rebuild the hydraulic

system.

Reports of DOT5 causing premature failure of rubber brake parts were more
common with early DOT5

formulations. This is thought to be due to improper addition of swelling
agents and has been fixed in recent

formulations.

Since DOT5 does not absorb water, any moisture in the hydraulic system
will "puddle" in one place. This can

cause localized corrosion in the hydraulics.

Careful bleeding is required to get all of the air out of the system. Small
bubbles can form in the fluid that will form

large bubbles over time. It may be necessary to do a series of bleeds.

DOT5 is slightly compressible (giving a very slightly soft pedal), and
has a lower boiling point than DOT4.

DOT5 is about twice as expensive as DOT4 fluid. It is also difficult to
find, generally only available at selected

auto parts stores.

DOT5.1

DOT5.1 is a relatively new brake fluid that is causing no end of confusion
amongst mechanics. The DOT could avoid a

lot of confusion by giving this new fluid a different designation. The
5.1 designation could lead one to believe that it's a

modification of silicone-based DOT 5 brake fluid. Calling it 4.1 or 6 might
have been more appropriate since it's a

glycol-based fluid like the DOT 3 and 4 types, not silicone-based like
DOT 5 fluid.

As far as the basic behavior of 5.1 fluids, they are much like "high performance"
DOT4 fluids, rather than traditional

DOT5 brake fluids.

Advantages:

DOT5.1 provides superior performance over the other brake fluids discussed
here. It has a higher boiling point,

either dry or wet, than DOT 3 or 4. In fact, its dry boiling point (about
275 degrees C) is almost as high as racing

fluid (about 300 degrees C) and 5.1's wet boiling point (about 175 to 200
degrees C) is naturally much higher

than racing's (about 145 C).

DOT5.1 is said to be compatible with all rubber formulations.

Disadvantages:

DOT5.1 fluids (and Spectro's Supreme DOT4) are non-silicone fluids and
will absorb water.

DOT5.1 fluids, like DOT3 & DOT4 will eat paint.

DOT 5.1 fluids are difficult to find for sale, typically at very few auto
parts stores, mostly limited to "speed shops."

DOT 5.1 will be more expensive than DOT3 or DOT4, and more difficult to
find.
 
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