By Andrew Markel
Editor, Brake & Front End
1. What are the fundamental differences between synthetic fluid and conventional DOT 3 and 4?
Simple, it all comes down to the base stock. Both synthetic
and conventional brake fluids start from the same “polyethylene glycol”
stock. But to make synthetic brake fluid the manufacturer will
original base stock and make the molecules better and more consistent.
could make the chains longer or add other molecules to enhance the
performance. Almost every synthetic brake fluid manufacturer does it
differently. To the base stock they add the additive package. This is a
mixture of anti-corrosion, anti-foaming and other secret chemicals that
give the fluid the desired performance characteristics for operation in
vehicles. DOT 5.1 is a high-performance certification (higher temps
than DOT 5). To meet the performance criteria, it takes a synthetic or
really good conventional base stock. Silicone is a synthetic substance.
But don’t confuse Dot 5 with synthetic polyethylene glycol-based fluids
like DOT 5.1.
2. What is the compatibility between synthetics and conventional DOT 3 and
4 in most vehicles? Is a complete flush with new rubber a better
choice? Is there any chance of seal swelling or disintegration in older
vehicles, say from the 1930s, 1940s or even in the 1960s?
DOT 3, 4 and 5.1 brake fluids are compatible with each other and with
all systems. All polyethylene glycol-based fluids will not harm healthy
rubber parts. Also, the additive packages will not damage or distort
any rubber parts. Even an older rubber part that has a high
concentration of natural rubber, will not be damaged by new DOT-rated
brake fluids. What kills rubber parts is when the additive package
additive package controls the pH of the fluid and the viscosity. If the
brake fluid becomes unable to control the pH or other corrosive
elements, the rubber and metal parts will deteriorate over time.
If you have an older vehicle and are worried about boiling the fluid, use DOT 4 or 5.1 brake fluid.
3. Can synthetics be mixed where DOT 5 silicone was used or is it recommended that the rubber be replaced when changing back?
is an inert substance. Technically it should not damage the rubber
parts. Polyethylene glycol based fluids (conventional or synthetic)
will not mix with silicone fluid (DOT 5). It will lump together
somewhere in the system. Also, they will not react when it is mixed.
So, if you perform a good flush, the rubber parts and the system should
4. Now that it is known that copper contamination is the main reason that brake fluids degrade, what property do synthetics have to address this problem?
comes from the copper brazing in the walls of the hard brake lines.
copper can be a problem for ABS and some valving. But it is an
indicator of the state of the brake fluid. The more copper that is
present, the greater the chance of corrosion happening in the system.
When there is corrosion, it is a sign that the brake fluid’s
anti-corrosion additives are depleted. If your base stock and additives
are of higher quality, chances are that they will not break down as
easily as low-quality ingredients.
5. Compared to conventional fluid, what life span can be expected from synthetics?
my opinion, it is about the same. Synthetic fluid might be able to last
a little longer because the fluid can absorb more water before the
fluid drops below a critical boiling point. But it is the additive
package and environment that has the greatest impact on life span, no
matter what type of base stock.
6. Do synthetic fluids provide a better pedal feel?
very high-end synthetic DOT 4 and 5.1 synthetic fluids can give a
stiffer pedal. But, it is so small of a difference that only the very,
very, very best drivers can feel it. It is mostly in their head. Some
silicone-based fluids can give a softer pedal because it is more
compressible than glycol-based fluids. But, some high-tech silicone
ester-based stuff that some race teams use is less compressible than
glycol-based fluids. But, this stuff can run $90 a liter! Also, it does
not have some critical corrosion inhibitors and is not DOT certified.
5 brake fluids still have their place. Owners of show cars can use the
fluid if they are concerned about damage to the paint.