ISO 4925:2020

INTERNATIONAL ISO
STANDARD 4925
Third edition
2020-07
Road vehicles — Specification of non-
petroleum-based brake fluids for
hydraulic systems
Véhicules routiers — Spécifications pour liquides de frein à base non
pétrolière pour systèmes hydrauliques
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Materials . 2
5 Specifications . 2
6 Test methods . 4
6.1 Viscosity . 4
6.1.1 General. 4
6.1.2 Repeatability (single analyst) . 4
6.1.3 Reproducibility (multi-laboratory). 4
6.2 Equilibrium reflux boiling point (ERBP) . 5
6.2.1 General. 5
6.2.2 Preparation of apparatus . 5
6.2.3 Test procedure . 5
6.2.4 Repeatability (single analyst) . 5
6.2.5 Reproducibility (multi-laboratory). 5
6.2.6 Wet ERBP test . 6
6.3 pH . 9
6.4 Fluid stability . 9
6.4.1 High-temperature stability . 9
6.4.2 Chemical stability .10
6.5 Corrosion .10
6.5.1 Metal strip characteristics prior to testing .10
6.5.2 Preparation of joints .10
6.5.3 Test procedure .11
6.6 Fluidity and appearance at low temperatures .11
6.6.1 At −40 °C for 144 h .11
6.6.2 At −50 °C for 6 h.11
6.7 Water tolerance .12
6.7.1 At −40 °C for 22 h .12
6.7.2 At 60 °C for 22 h.12
6.8 Compatibility/miscibility with ISO 4926 fluid .12
6.8.1 At −40 °C for 22 h .12
6.8.2 At 60 °C for 22 h.12
6.9 Resistance to oxidation .12
6.10 Effect on rubber .13
6.10.1 Test procedures.13
6.10.2 Repeatability (single analyst) .14
6.10.3 Reproducibility (multi-laboratory).14
6.11 Reserve alkalinity according to ASTM D 1121 .14
Annex A (normative) ISO styrene-butadiene rubber (SBR) brake cups for testing brake fluid .15
Annex B (normative) Corrosion test strips .18
Annex C (informative) Corrosion strip assembly .19
Annex D (normative) Standard ethylene, propylene and diene (EPDM) terpolymer rubber
slabstock .20
Annex E (normative) Triethylene glycol monomethyl ether (TEGME) brake fluid grade .22
Bibliography .25
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee 33
Chassis systems and components.
This third edition cancels and replaces the second edition (ISO 4925:2005), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— a terms and definitions clause was added (Clause 3);
— an additional class 7 is introduced and added to the already existing classes 3, 4, 5-1 and 6;
— the reserve alkalinity is added upon request out of the automotive industry;
— the intended introducing of a stroking test into a future revision of this document is replaced by the
intended introduction of a wear and noise test currently under development in the SAE and ISO TF
lubrication.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved

Introduction
The specifications for fluids given in this document incorporate a range of performance standards in
use throughout the world at the time of publication. To the already existing classes 3, 4, 5-1 and 6 an
additional class 7 is added.
The major use of these fluids is in the hydraulic brake and clutch systems of road vehicles, but they can
also be used in any suitable hydraulic system.
INTERNATIONAL STANDARD ISO 4925:2020(E)
Road vehicles — Specification of non-petroleum-based
brake fluids for hydraulic systems
1 Scope
This document provides the specifications, requirements and test methods, for non-petroleum-based
fluids used in road-vehicle hydraulic brake and clutch systems that are designed for use with such fluids
and equipped with seals, cups or double-lipped type gland seals made of styrene-butadiene rubber
(SBR) and ethylene-propylene elastomer (EPDM).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 37, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties
ISO 48-2, Rubber, vulcanized or thermoplastic — Determination of hardness (hardness between 10 IRHD
and 100 IRHD)
ISO 812, Rubber, vulcanized or thermoplastic — Determination of low-temperature brittleness
ISO 815 (all parts), Rubber, vulcanized or thermoplastic — Determination of compression set
ISO 3104, Petroleum products — Transparent and opaque liquids — Determination of kinematic viscosity
and calculation of dynamic viscosity
ISO 4926, Road vehicle — Hydraulic braking systems — Non petroleum base reference fluids
ASTM D 91, Standard test method for precipitation number of lubricating oils
ASTM E 298, Standard test methods for assay of organic peroxides
ASTM D 395, Standard test methods for rubber property — Compression set
ASTM D 412, Standard test methods for vulcanized rubber and thermoplastic elastomers — Tension
ASTM D 746, Standard test method for brittleness temperature of plastics and elastomers by impact
ASTM D 865, Standard test method for rubber — Deterioration by heating in air (test tube enclosure)
ASTM D 1120, Standard test method for boiling point of engine coolants
ASTM D 1121, Standard test method for reserve alkalinity of engine coolants and antirusts
ASTM D 1123, Standard test methods for water in engine coolant concentrate by the Karl Fisher
reagent method
ASTM D 1415, Standard test method for rubber property — International hardness
ASTM D 3182, Standard practice for rubber — Materials, equipment and procedures for mixing standard
compounds and preparing standard vulcanized sheets
ASTM D 3185:2006, Standard test methods for rubber—Evaluation of SBR (Styrene-Butadiene Rubber)
including mixtures with oil
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Materials
On visual inspection, the fluid shall be clear and free of suspended matter, dirt and sediment. The
quality of the materials used shall be such that the resulting product conforms to the requirements
of this document and that uniformity of performance is ensured. Fluids may be dyed, provided no
confusion is possible between them and other types of fluids.
5 Specifications
The product shall meet the requirements for the appropriate class in accordance with Table 1, using the
test methods according to Clause 6.
Table 1 — Brake fluid specifications — Tests and requirements
Requirement(s)
Test
method Test description Unit
Class Class Class Class Class
(subclause)
3 4 5–1 6 7
6.1 Viscosity
at −40 °C mm /s ≤1 500 ≤900 ≤750 ≤750
at 100 °C mm /s ≥1,5
6.2 Equilibrium reflux boiling point (ERBP) °C ≥205 ≥230 ≥260 ≥250 ≥260
6.2.6 Wet ERBP °C ≥140 ≥155 ≥180 ≥165 ≥180
6.3 pH — 7 to 11,5
6.4 Fluid stability
6.4.1 High-temperature stability °C ±5 °C
6.4.2 Chemical stability °C ±5 °C
6.5 Corrosion
Metal strip characteristics after testing
Mass change
Tinned iron mg/cm −0,2 to 0,2
Steel mg/cm −0,2 to 0,2
Aluminium mg/cm −0,1 to 0,1
Cast iron mg/cm −0,2 to 0,2
Brass mg/cm −0,4 to 0,4
Copper mg/cm −0,4 to 0,4
Aspect — No pitting or roughness outside
contact area
Staining/discoloration — Permitted
Liquid characteristics after testing
NOTE It is intended that a “wear” and “noise” lubrication test, currently under development in the SAE and ISO TF
lubrication, will be added to a future revision of this document.
NOTE Reserve alkalinity is requested by many customers.
2 © ISO 2020 – All rights reserved

Table 1 (continued)
Requirement(s)
Test
method Test description Unit
Class Class Class Class Class
(subclause)
3 4 5–1 6 7
Aspect — No gel, none adhering crystals
pH — 7 to 11,5
Sediment % vol. ≤0,1
Rubber cup characteristics after testing
Blisters or carbon black separation — None
at surface
Hardness decrease IRHD ≤15
Base diameter increase mm ≤1,4
Volume increase % ≤16
6.6 Fluidity and appearance at low temperatures
6.6.1 at −40 °C for 144 h
Aspect — Clear and homogeneous
Bubble flow time s ≤10
Sediments — Absence
6.6.2 at −50 °C for 6 h
Aspect — Clear and homogeneous
Bubble flow time s ≤35
Sediments — Absence
6.7 Water tolerance
6.7.1 at −40 °C for 22 h
Aspect — Clear and homogeneous
Bubble flow time s ≤10
Sediments — Absence
6.7.2 at 60 °C for 22 h
Aspect — Clear and homogeneous
Sediments % vol. ≤0,05
6.8 Compatibility/miscibility with ISO 4926 fluid
6.8.1 at − 40 °C for 22 h
Aspect — Clear and homogeneous
Sediments — Absence
6.8.2 at 60 °C for 22 h
Aspect — Clear and homogeneous
Sediments % vol. ≤0,05
6.9 Resistance to oxidation
Metal strip aspect No pitting or roughness
—
no more than a trace of gum
Staining/discoloration — Permitted
Mass change of aluminium strip mg/cm −0,05 to +0,05
Mass change of cast iron strip mg/cm −0,3 to +0,3
6.10 Effect on rubber
NOTE It is intended that a “wear” and “noise” lubrication test, currently under development in the SAE and ISO TF
lubrication, will be added to a future revision of this document.
NOTE Reserve alkalinity is requested by many customers.
Table 1 (continued)
Requirement(s)
Test
method Test description Unit
Class Class Class Class Class
(subclause)
3 4 5–1 6 7
6.10.1.1 Styrene butadiene rubber (SBR)
at 120 °C
Cup diameter increase mm 0,15 to 1,4
Hardness change IRHD −15 to 0
Volume increase % 1 to 16
Blisters or carbon black separation at None
—
surface
6.10.1.2 Ethylene propylene diene monomer (EPDM)
at 120 °C
Hardness change IRHD −15 to 0
Volume change % 0 to 10
Blisters or carbon black separation at None
—
surface
6.11 Reserve alkalinity according to ASTM D 1121 mL to be reported
NOTE It is intended that a “wear” and “noise” lubrication test, currently under development in the SAE and ISO TF
lubrication, will be added to a future revision of this document.
NOTE Reserve alkalinity is requested by many customers.
6 Test methods
6.1 Viscosity
6.1.1 General
Determine the kinematic viscosity of the fluid in accordance with ISO 3104.
2 2
Report the viscosity to the nearest 1 mm /s at −40 °C and to the nearest 0,01 mm /s at +100 °C.
Duplicate runs that agree within 1,2 % relative are acceptable for averaging (95 % confidence level).
6.1.2 Repeatability (single analyst)
The coefficient of variation of results (each the average of duplicates) obtained by the same analyst
on different days shall not be greater than 0,4 % at 47 degrees of freedom. Two such values shall be
considered unacceptable (95 % confidence level) if they differ by more than 1,2 %.
6.1.3 Reproducibility (multi-laboratory)
The coefficient of variation of results (each the average of duplicates) obtained by analysts in different
laboratories shall not be greater than 1,0 % at 15 degrees of freedom. Two such values shall be
considered unacceptable (95 % confidence level) if they differ by more than 3,0 %.
4 © ISO 2020 – All rights reserved

6.2 Equilibrium reflux boiling point (ERBP)
6.2.1 General
1)
Determine the ERBP of the fluid in accordance with ASTM D 1120 , but with the following changes to
the procedure and to the apparatus (see Figures 1 and 2).
— Thermometer: immersion shall be 76 mm and the thermometer shall be calibrated.
— Heat source: use either a suitable variac-controlled heating mantle designed to fit the flask, or an
electric heater with rheostat heat control.
6.2.2 Preparation of apparatus
Thoroughly clean and dry all glassware before use. Attach the flask to the condenser. When using a
heating mantle, place the mantle under the flask and support it with a suitable ring clamp and laboratory-
type stand, holding the whole assembly in place by a clamp. When using a rheostat-controlled heater,
centre a standard porcelain or other suitable refractory having a diameter opening of 32 mm to 38 mm
over the heating element of the electric heater and mount the flask on the refractory so that direct heat
is applied to the flask only through the opening in the refractory. Place the whole assembly in an area
free from draughts or other causes of sudden temperature changes.
6.2.3 Test procedure
Turn on the condenser water and apply heat to the flask at such a rate that the fluid is refluxing within
(10 ± 2) min at a rate higher than one drop per second. The reflux rate shall not exceed five drops per
second. Immediately adjust the heat input to obtain a specified equilibrium reflux rate of one drop
per second to two drops per second over the next (5 ± 2) min period. Maintain a timed and constant
equilibrium reflux rate of one drop per second to two drops per second for an additional 2 min; record the
average value of four temperature readings taken at 30 s intervals at the equilibrium reflux boiling point.
Report the boiling point to the nearest degree Celsius. Duplicate results that agree within 3 °C are
acceptable for averages (95 % confidence level).
6.2.4 Repeatability (single analyst)
The standard deviation of results (each the average of duplicates), obtained by the same analyst on
different days shall not be greater than 1,3 °C at 34 degrees of freedom. Two such values shall be
considered unacceptable (95 % confidence level) if they differ by more than 4 °C.
6.2.5 Reproducibility (multi-laboratory)
The standard deviation of results (each the average of duplicates), obtained by analysts in different
laboratories, shall be not greater than 3,5 °C at 15 degrees of freedom. Two such values shall be
considered unacceptable (95 % confidence level) if they differ by more than 10,5 °C.
1) Boiling chips for use with ASTM D 1120 can be obtained from Electro Minerals Co. (US) Inc, PO Box 423, Niagara
Falls, NY 14302, USA, or from the Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale
Pa 15096, USA (RM-75). Boiling chip is an example of a suitable product available commercially. This information is
given for the convenience of users of this document and does not constitute an endorsement by ISO of this product.
6.2.6 Wet ERBP test
6.2.6.1 Apparatus
5.2.6.1.1 Two corrosion test jars or equivalent screw-top, straight-sided, round glass jars each having
a capacity of about 475 ml and approximate inner dimensions of 100 mm height by 75 mm diameter,
2)
with matching lids having new, clean inserts providing water-proof and vapour-proof seals .
5.2.6.1.2 Desiccator and cover: bowl-form glass desiccator with 250 mm inside diameter, having a
matching tubulated cover fitted with a No. eight rubber stopper (see Figure 3).
5.2.6.1.3 Desiccator plate of 230 mm diameter, perforated porcelain desiccator plate, without feet,
3)
glazed on one side (No.18 or equivalent) .
2) Suitable corrosion test jars (RM-49) and tinned steel lids (RM-63) can be obtained from the Society of Automotive
Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pa 15096, USA.
3) Desiccator plates (No. 08-641C) can be obtained from Fischer Scientific, Springfield, New Jersey, USA or
CeramTec AG (No. 602786), Glaswerk Wertheim KG (No. 911743431) or equivalents, according to DIN/ISO 12911,
diameter 235 mm.
6 © ISO 2020 – All rights reserved

Dimensions in millimetres
Key
1 water outlet
2 water jacket
3 thermometer
4 plastic screw cap or rubber sleeve
5 boiling chips
6 19/38 joint
7 water inlet
Figure 1 — Boiling point test apparatus
Dimensions in millimetres
Key
1 19/38 joint
2 fire-polished
3 screw joint or rubber sleeve
a
Internal diameter: 8 to 9.
Figure 2 — Detail of 100 ml short-neck flask
6.2.6.2 Test procedure
To determine the wet ERBP of the fluid in duplicate (see Figure 3), first, humidify a 350 ml sample of
the fluid under controlled conditions, using 350 ml of triethylene glycol mono methyl ether (TEGME),
follow Annex E, to establish the end point for humidification.
Lubricate the ground-glass joint of the desiccator.
Then, pour (450 ± 10) ml of distilled water into the desiccator and insert the perforated porcelain plate.
Immediately place one open corrosion test jar containing (350 ± 5) ml of the humidified test fluid into
the desiccator. Place a second open corrosion test jar containing (350 ± 5) ml of TEGME control fluid
into the same desiccator. The water content of the TEGME control fluid at the start of exposure shall be
(0,50 ± 0,05) % by weight.
Next, replace the desiccator cover and place immediately in a forced ventilation oven set at (50 ± 1) °C.
Periodically, during oven humidification, remove the rubber stopper from the desiccator and, using a
long-needle hypodermic syringe, quickly sample the control fluid and determine its water content in
accordance with ASTM D 1123. A maximum of 10 ml of fluid shall be removed in total. When the water
content of the control fluid has reached (3,70 ± 0,05) % by weight, remove the desiccator from the oven
and seal the test jar promptly using a screw-cap jar lid. Allow the sealed jar to cool for 60 min to 90 min
at (23 ± 5) °C. Determine the ERBP in accordance with 6.2.1 and 6.2.3.
8 © ISO 2020 – All rights reserved

Dimensions in millimetres
Key
1 rubber stopper
2 glass desiccator with tubulated cover
3 lubricated ground joint
4 TEGME (see Annex E)
5 corrosion test jars
6 fluid sample
7 porcelain desiccator plate
8 water
Figure 3 — Humidification apparatus
6.3 pH
Mix the fluid with an equal volume of a mixture 50 % ethanol and 50 % distilled water neutralized to
a pH of 7,0 ± 0,1. Determine the pH of the resulting solution electrometrically at (23 ± 5) °C, using a pH
meter equipped with a calibrated full range (0 to 14) glass electrode and a calomel reference electrode,
as specified in ASTM D 664.
Clouding or muddiness of the resulting solution is permitted in the determination.
6.4 Fluid stability
6.4.1 High-temperature stability
To determine the high-temperature stability of the fluid, heat a fresh sample of the original test fluid
to a temperature of (185 ± 2) °C according to the procedure specified in 6.2.3 and maintain it at that
temperature for (120 ± 5) min before determining the boiling point of the fluid, also in accordance with
6.2.3. The difference between this observed boiling point and that previously determined using that
test procedure shall be considered as the change in boiling point of the fluid.
6.4.2 Chemical stability
To determine the chemical stability of the fluid, first, mix 30 ml of the fluid with 30 ml of the fluid
specified in ISO 4926. Determine the ERBP of this fluid mixture by use of the test apparatus specified
in 6.2, applying heat to the flask at such a rate that the fluid refluxes within (10 ± 2) min at a rate of
between one drop per second and five drops per second.
Record the maximum fluid temperature observed during the first minute after the fluid begins refluxing
at a rate higher than one drop per second. Over the next (15 ± 1) min, adjust and maintain the rate of
reflux to one drop per second to two drops per second. Maintain a timed and constant equilibrium
reflux rate of from one drop per second to two drops per second for an additional 2 min, recording
the average value of four temperature readings at 30 s intervals as the final equilibrium reflux boiling
point. Chemical reversion is eviden
...