Overview - ISO 4925:2020 (non‑petroleum‑based brake fluids)
ISO 4925:2020 specifies performance, material and test requirements for non‑petroleum‑based brake fluids used in road‑vehicle hydraulic brake and clutch systems. The third edition (2020) updates the previous 2005 version, adds a new Class 7, and introduces reserve alkalinity as an assessed property. The standard applies where systems use seals, cups or double‑lipped gland seals made of styrene‑butadiene rubber (SBR) and ethylene‑propylene elastomer (EPDM).
Key technical topics and requirements
ISO 4925 sets out required properties, test methods and acceptance limits for brake fluids, including:
- Viscosity: low‑temperature and high‑temperature viscosity criteria (e.g., limits at −40 °C and 100 °C) to ensure reliable hydraulic response in cold and hot conditions.
- Equilibrium Reflux Boiling Point (ERBP) and Wet ERBP: minimum boiling points by class (classes 3, 4, 5‑1, 6 and 7) to reduce vapor lock risk.
- pH and reserve alkalinity: acceptable pH range and alkalinity checks to monitor fluid stability and corrosion protection.
- Fluid stability: high‑temperature and chemical stability tests to verify additive and base‑fluid durability.
- Corrosion and oxidation resistance: metal strip tests (aluminium, cast iron, steel, copper, brass, tinned iron) to limit mass change, pitting and surface roughness.
- Low‑temperature fluidity and appearance: bubble flow time and sediment checks at −40 °C and −50 °C.
- Water tolerance: performance after controlled water content exposure and stability at elevated temperature.
- Compatibility/miscibility: mixing behavior with the ISO 4926 reference fluid and absence of sediment or phase separation.
- Effect on rubber: SBR and EPDM elastomer tests for hardness, swelling and surface deterioration.
The standard defines detailed test methods (Clause 6) and normative annexes for test materials (SBR cups, EPDM slabstock, corrosion strips) and reference fluids.
Practical applications - who uses ISO 4925
ISO 4925 is used by:
- Brake fluid manufacturers - to formulate and certify non‑petroleum brake fluids to global performance classes.
- Automotive OEMs and suppliers - to specify acceptable fluids for brake/clutch systems and compatible seal materials (SBR, EPDM).
- Test laboratories and certification bodies - to run standardized ERBP, viscosity, corrosion, oxidation and elastomer compatibility tests.
- Aftermarket and fleet managers - to select fluids that meet service and safety requirements (cold performance, boiling point, water tolerance).
- Regulators and standards committees - for harmonized safety and performance benchmarks.
Related standards
- ISO 4926 - non‑petroleum base reference fluids for hydraulic braking systems
- Relevant ASTM methods referenced in ISO 4925 (e.g., ASTM D1121 for reserve alkalinity, ASTM methods for water and rubber testing)
ISO 4925:2020 is essential for ensuring the safety, reliability and compatibility of non‑petroleum‑based brake fluids across a wide range of road‑vehicle hydraulic systems.
Open preview (PDF)Road vehicles — Specification of non-petroleum-based brake fluids for hydraulic systems
