1.1) Hydraulic fluid:
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is
transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or
water. Examples of equipment that might use hydraulic fluids
include  excavators  and  backhoes,  hydraulic brakes,  power steering  systems,
transmissions,  garbage trucks,  aircraft flight control systems,  lifts,  and industrial
machinery. s like the ones mentioned above will work most efficiently if the hydraulic
fluid. Hydraulic system used has zero compressibility.
1.1.1) Functions and properties:
 The primary function of a hydraulic fluid is to convey power. In use, however,
there are other important functions of hydraulic fluid such as protection of the hydraulic
machine components. The table below lists the major functions of a hydraulic fluid and
the properties of a fluid that affect its ability to perform that function.
Table 1.1) Functions and properties of Brake Fluids.
Function Property
  Fast air release
  Low foaming tendency
  Sealing Medium   Adequate viscosity and viscosity index 
  Shear stability
  Low temperature fluidity
leakage
  Biodegradability 
1.1.2.1) Base stock:
 The original hydraulic fluid, dating back to the time of  ancient Egypt, was water. 
Beginning in the 1920s, mineral oil began to be used more than water as a base stock
due to its inherent lubrication properties and ability to be used at temperatures above
the boiling point of water. Today most hydraulic fluids are based on mineral oil base
stocks.
Natural oils such as rapeseed (also called canola oil) are used as base stocks for
fluids where biodegradability and renewable sources are considered important.
Other base stocks are used for specialty applications, such as for fire resistance
and extreme temperature applications. Some examples include: glycol, esters,
organophosphate ester, polyalphaolefins, propylene glycol, and silicone oils.
1.1.2.2) Other Components:
Hydraulic fluids can contain a wide range of chemical compounds, including oils, 
butanol,  esters (e.g.  phthalates,  like  DEHP,  and  adipates,  like  bis(2-ethylhexyl)
adipate), polyalkylene glycols (PAG), organophosphate (e.g. tributylphosphate), silicones,
inhibitors (incl acid scavengers), anti-erosion additives, etc.
1.1.3) Trade names:
  Arnica,
  Tellus,
  Durad,
  Fyrquel,
  Houghto-Safe,
  Hydraunycoil,
1.2.1) Background:
One of the most important hydraulic applications on the modern automobile is
the brake system. The first motor vehicle having hydraulic brake system employed raw
hide seals and flexible hoses to transmit hydraulic pressure from the master cylinder to
the cylinders on the front wheels.
Non-Petroleum hydraulic fluids which have no effect on rubber and leather were used
in these systems. This selection set the precedent for the type of brake fluids that still
exist today.
 The earliest brake fluids were solution of sugar and water or glycerin with water
or alcohol. However all of these fluids were unsuitable because they caused corrosion
and gumming of brake parts. Rapid deterioration of the pistons and cylinders resulted
in the leakage of the fluid. Leather seals were replaced by natural rubber brake cups and
improved fluids of castor oils and alcohols replaced the older fluids, but corrosion and
gum problems, although lessened, persisted due to formulation of acidic oxidation and
hydrolysis of castor oil. The addition of cystic was found use full as a partially effected
corrosion inhibitor for these fluids.
In order to keep up with the increase temperature demands of the newer brake
system, higher alcohol such as butanol and diacetone alcohol gradually supplanted the
ethanol and water type fluids.
Ethylene glycol monoethylene ether was the first glycol used in brake fluids this
diluent started the trend towards the higher boiling points fluids. The use non
crystalizing poly glycol lubricants made possible the preparation of brake fluids capable
of operating at extremely low temperature. Fluids implying glycol ethers such as cello
solve and carbitol (Diethylene glycol mono ethyl ether) as diluents, with glycols as
lubricants, made it possible to operate at ambient temperatures ranging from 125oF to -
40oF,
1.2.2) Definition: 
Brake fluid  is a type of   hydraulic fluid  used in  hydraulic brake  and  hydraulic
clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used
to transfer force into pressure, and to amplify braking force. It works because liquids are
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not appreciably compressible  in their natural state the component molecules do not
have internal voids and the molecules pack together well, so bulk forces are directly
transferred to compress the fluid's chemical bonds.
1.2.3) Classification:
Most brake fluids used today are glycol-ether based, but mineral oil (Citroën liquid
hydraulique mineral (LHM) and silicone (DOT 5) based fluids are also available. 
1.2.4) Standard Specifications:
Brake fluids must meet certain requirements as defined by various standards set
by organizations such as the SAE, or local government equivalents. For example, most
brake fluid sold in North America is classified by the US  Department of
 Transportation  (DOT) under its own ratings such as "DOT 3"  and "DOT 4". Their
classifications broadly reflect the concerns addressed by the SAE's specifications, but
with local details -  Alaska  and the  Azores  have different normal temperature and
humidity ranges to consider, for example; many countries defer explicitly to the SAE
specifications, or simply refer to "best practice" which in application would defer to SAE
standard.
Table 1.2) TYPICAL TECHNICAL PROPERTIES of Break-In Oil SAE 30) - BRK 
TYPICAL TECHNICAL PROPERTIES of Break-In Oil SAE 30) BRK SAE 30
Kinematic Viscosity at 100oC, cSt (ASTM D-445) 11.4
Kinematic Viscosity at 40oC, cSt (ASTM D-445) 91.6
Viscosity Index (ASTM D-2270) 112
Flash Point oC (oF) (ASTM D-92) 236 (457)
Fire Point oC (oF) (ASTM D-92) 254 (489)
Pour Point oC (oF) (ASTM D-97) -36 (-32)
Four-Ball Wear (ASTM D-4172)
Para 1 (40 kg, 75ºC, 1200 rpm, 1 hr), Scar, mm 0.45
High-Temperature/High-Shear Viscosity (ASTM D-5481 at 150ºC, 1.0 x
106 S-1), CP 3.5
Brake fluids must have certain characteristics and meet certain quality standards
for the braking system to work properly.
1.2.5.1) Boiling point:
Brake fluid is subjected to very high temperatures, especially in the  wheel
cylinders of drum brakes and disk brake calipers. It must have a high boiling point to
avoid vaporizing in the lines. This vaporization is a problem because vapor is highly
compressible relative to liquid, and therefore negates the hydraulic transfer of braking
force.
Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet
boiling point, which is usually much lower (although above most normal service
temperatures), refers to the fluid's boiling point after absorbing a certain amount of
moisture. This is several (single digit) percent, varying from formulation to formulation.
Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which
means they absorb moisture from the atmosphere under normal humidity levels. Non-
hygroscopic fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can
maintain an acceptable boiling point over the fluid's service life, although at the cost of
potential phase separation/water pooling and freezing/boiling in the system over time -
the main reason single phase hygroscopic fluids are used.
Table 1.3) Characteristics of common braking fluids: 
Type Dry boiling point  Wet boiling
point
Viscosity limit Primary constituent
DOT 2 190 oC (374 oF) 140 oC (284 oF) ? Castor oil/alcohol
DOT 3 205 oC (401 oF) 140 oC (284 oF) 1500 mm2/s Glycol
Ether/Benzoate Ester
DOT 4 230 oC (446 oF) 155 oC (311 oF) 1800 mm2/s Glycol Ether/Borate
Ester
DOT 5 260 oC (500 oF) 180 oC (356oF) 900 mm2/s Silicone
DOT 5.1 260 oC (500 oF) 180 oC (356 oF) 900 mm2/s Glycol Ether/Borate
Ester
1.2.5.2) Viscosity:
For reliable, consistent brake system operation, brake fluid must maintain a
constant viscosity under a wide range of temperatures, including extreme cold. This is
especially important in systems with an anti-lock braking system (ABS), traction control, and
stability control (ESP), as these systems may use a valve with a time-based approach,
rather than measuring pressure or volume to control the amount of fluid transferred.
1.2.5.3) Corrosion:
Brake fluids must not corrode the metals used inside components such as calipers,
wheel cylinders, master cylinders and ABS control valves. They must also protect against
corrosion as moisture enters the system. Additives (corrosion inhibitors) are added to
the base fluid to accomplish this.
 The advantage of the Citroën LHM mineral oil based brake fluid is the absence of
corrosion. Seals may wear out at high mileages but otherwise these system have
exceptional longevity. It cannot be used as a substitute without changing seals due to
incompatibility.
1.2.5.4) Compressibility:
Brake fluids must maintain a low level of compressibility that remains low, even
with varying temperatures.
1.2.6) Service and maintenance:
Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT
4, and DOT 5.1) should be flushed, or changed, every 1 2 years under non-racing
conditions. Many manufacturers also require periodic fluid changes to ensure reliability
and safety. Once installed,  moisture  diffuses  into the fluid through brake hoses and
rubber seals and, eventually, the fluid will have to be replaced when the water content
becomes too high. Electronic testers and test strips are commercially available to
measure moisture content, however moisture test strips were taken off the market
because they absorb moisture in the air before they can be used. The corrosion inhibitors
also degrade over time. Degraded inhibitors cause corrosion in the braking system. The
first metal to corrode is copper. You can determine when it is time to replace brake fluid
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when copper ions hit 200ppm. New fluid should always be stored in a sealed container
to avoid moisture intrusion.
DOT 5 is silicone fluid and the above does not apply. Ideally, silicone fluid should
be used only to fill non-ABS systems that have not been previously filled with glycol
based fluid. Any system that has used glycol based fluid will contain moisture; glycol
fluid disperses the moisture throughout the system and contains corrosion inhibitors.
Silicone fluid does not allow moisture to enter the system, but does not disperse any that
is already there, either. A system filled from dry with silicone fluid does not require the
fluid to be changed at intervals, only when the system has been disturbed for a
component repair or renewal. The United States armed forces have standardized on
silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climate,
particularly in Russia and Finland.
A small drop in brake fluid level in the master cylinder reservoir can be "topped
up" but if the level consistently drops, the cause should be investigated and repaired.
Brake fluid level in the master cylinder will drop as the linings (pads or shoes) wear and
the calipers or wheel cylinders extend further to compensate. Overspill from pushing
back pistons should be avoided, because glycol based fluid will quickly lift or strip paints
and other coatings on contact (it can be removed by quickly washing with water, not
wiping). Brake fluid level may also be low because of a leak, which could result in a loss
of hydraulic pressure and consequently a significant loss of braking ability. Modern cars
have  redundant  hydraulic circuits (two separate circuits) to ensure against total
hydraulic failure.
1.2.7) Mixing and replacement:
Brake fluids with different DOT ratings cannot always be mixed. It must be of the
same type, and at least the same rating. DOT 5.1 can replace DOT 4 and 3, DOT 4 can
replace DOT 3. DOT 5 should not be mixed with any of these as mixing of glycol with
silicone fluid may cause corrosion because of trapped moisture.
1.2.8.) Toxicity:
Brake fluid is toxic and combustible and can damage painted surfaces.
  Castor oil
  Alcohol, usually butanol (red / crimson fluid) or ethanol (yellow fluid)
b) Glycol-based DOT 3, 4, 5.1)
  Alkyl ester
  Aliphatic amine
  Diethylene glycol