Brake Fluid for Classic & Vintage Cars
A Little History
In the 1920's, when automobiles progressed from mechanical brakes actuated by cables and levers to hydraulic brakes, the only material available for flexible hoses was natural rubber. As a result, a hydraulic fluid compatible with natural rubber was needed and a mixture of castor oil and alcohol was found to do the job. Today's brake fluids are so vastly superior to the old fluids originally specified for most of our older vintage and classic cars that it makes little sense to use the older fluid types today. The most notable advances have been in raising boiling points, improving compatibility with other brake fluids, reducing moisture absorption and reducing corrosion.
Brake System Fundamentals
The main function of brake fluid is to transmit the force applied to the brake pedal to the brake pads and shoes. To do this efficiently, brake fluids must be non-compressible. In addition, they must also not boil, thicken or freeze neither corrode nor chemically react with any materials in the hydraulic system. They must lubricate internal moving parts, flow easily through small passages, have a long and stable shelf life, and be compatible with other brake fluids. When brakes are applied on a moving car, the kinetic energy of the vehicle in motion is converted into heat. The faster the car is moving and the faster it is stopped, the more heat is produced.
Traditional Concerns for Vintage and Classic Car Owners
In the good old days, dire warnings of problems caused by using the wrong brake fluid in a vintage car were common. Such strong concerns were very valid in the 1950s, but much less so now. There are several reasons why we can be less worried about our hydraulic systems "turning to goo" if the wrong (modern-day) fluid is used:
1) Pure natural rubber hydraulic seals are no longer made for our cars.
2) More than likely, the original natural rubber seals have been replaced.
3) Brake fluids meeting current standards are compatible with virtually every type of seal available, including natural rubber.
4) Brake fluids meeting current standards are safe to mix (although mixing them is not recommended)
Classification of Brake Fluids
Brake fluids are classified by their physical properties. The standards in the US come from the Department of Transportation, which is where the “DOT” in the brake fluid classification DOT 3, DOT4, etc. comes from.
The different chemical bases currently used to make brake fluid are polyalkylene glycol ether (commonly called glycol), silicone, and mineral oil. A few vehicles, including Citroen and Rolls Royce, use a mineral or petroleum oil based central hydraulic system which also powers the brakes. To do this the brake system is fitted with special rubber components that are compatible with petroleum products. The special oil currently being used is “Liquide Hydraulique Minéral”, generally called LHM. It is dyed green for identification and it is NOT compatible with conventional brake systems, nor are conventional brake fluids compatible with systems requiring LHM. At the risk of offending the Citroen and Roll Royce owners, this article is focuses on the glycol and silicone based fluids. DOT 3, 4, 5.1 and 6 brake fluids are glycol-based. DOT 4 has borate esters which the DOT 3 does not, and DOT 5 fluids are silicone based.
Federal Motor Vehicle Safety Standard 116
“The purpose of FMVSS 116 is to reduce failures in the hydraulic braking systems of motor vehicles which may occur because of the manufacture or use of improper or contaminated fluid.” The FMVSS 116 details standards and the testing procedures used to measure the properties of brake fluid. Note the inclusion of natural rubber, the material used to make the original seals and cups in vintage and classic cars.
The tests are numbered 6.1 to 6.13, and are detailed below:
6.1 Equilibrium Reflux Boiling Point (ERBP)
Also known as “dry boiling point”. The term reflux simply means the fluid is kept at the boiling temperature with a water-cooled condenser to return the solvent vapours to the reaction vessel and prevent their escape. This allows the boiling point to be measured with great accuracy.
6.2 Wet ERBP
Also known as “wet boiling point”. This test measures the boiling point of “humidified brake fluid.” Different brake fluids absorb water at different rates; brake fluid that absorbs more water during the test will have a lower boiling point than another. This test is intended to approximate the amount of water contamination that might be absorbed in a year of use in a brake system under average conditions.
6.3 Kinematic Viscosity
This consists of a series of tests that determine the flow rate of the brake fluid under controlled temperatures. This is done to ensure that the fluid does not thin out too much at high temperatures, or thicken up too much at low temperatures. It is important to realize that the three attributes discussed so far- dry boiling point, wet boiling point and viscosity are the only standards that vary from one DOT class of brake fluid to another. Every other standard applies to every brake fluid, with some noted exceptions for silicone fluids.
6.4 ph Value
The pH is a measure of the acidity of the brake fluid. The standard calls for a pH not less than 7.0 or more than 11.5, 7 being neutral, and numbers over 7 being increasingly basic. Acidity is a factor in corrosion. This standard does not apply to silicone based fluids.
6.5 Fluid Stability
The effect of prolonged heating under reflux is tested by measuring the change in the boiling point. This ensures that the ERBP of brake fluid that has been boiled by heavy braking will not change significantly. It also ensures that mixing approved brake fluids will not significantly change the ERBP.
Six specified metal corrosion test strips (steel, tinned iron, cast iron, aluminium, brass, and copper) are polished, cleaned, and weighed, then placed on a standard rubber wheel cylinder cup in a corrosion test jar. Corrosion inhibitors used in brake fluid are typically based on a chemical group called ‘amine.’ The amine-based inhibitors are well known as being able to protect iron or steel from corrosion in aggressive high-temperature liquid environments. These corrosion inhibitors are included to protect the various metal components of the brake system from the corrosion that would be caused by the water that is absorbed by the brake fluid over time.
6.7 Fluidity and Appearance
Brake fluid is chilled to expected minimum exposure temperatures (-40 and -58º F) and observed for clarity, the formation of gel, sediment, separation of components, excessive viscosity or thixotropy. Although it is nice to know if the fluid is still functional at these extremes, it is not something Australia would be concerned about.
6.8 Water Tolerance
Sample glycol based brake fluid is diluted with water (to 3.5%). Silicone brake fluid is humidified and the samples are stored for 120 hours at -40 º F. The fluid will fail if there is any evidence of sludging, sedimentation, crystallization, or stratification. The sample is then heated to 140º F for 24 hours and the examination repeated. The effect of both the cold and heat cycle on the movement of an air bubble through the test fluid is also checked; if the bubble moves too slowly through the fluid, it fails.
This is a repeat of the Water Tolerance test, although in this case the water has been replaced by an SAE test brake fluid with known chemistry. This test ensures that fluids meeting the DOT specifications can be mixed without degrading the performance of the brake fluid.
6.10 Resistance to Oxidation
Glycol based brake fluids are activated with a mixture of approximately 0.2 percent benzyl peroxide and 5 percent water. Various strips of metal are subjected to exposure to the fluids and the air. At the end of the test, the metal strips are examined for pitting, etching, and loss of mass.
6.11 Effect on Styrene-Butadiene Rubber (SBR) Wheel Cylinder Cups
This test measures the effects of a brake fluid in swelling, softening, and otherwise affecting standard wheel cylinder cups. Too much swelling can have adverse effects because the proper operation of the master cylinder can be impaired if ports and orifices are covered by seals or cups that have increased in size. The seals in replacement brake master cylinders, wheel cylinders and callipers are SBR or EPDM rubber.
Water Contamination & Boiling Point of Glycol Based Brake Fluids
Glycol-based brake fluids are hygroscopic, meaning they absorb water from the atmosphere. Water contamination may cause corrosion of brake cylinder bores and pistons, and may seriously affect braking efficiency and safety of the brake actuating system.
How Does the Water Get In?
Most comes from the vent in the master cylinder cap and resultant condensation in the air space above the fluid. If cans of brake fluid and master cylinders are allowed to remain open to the atmosphere for too long, it will greatly increase the amount of water absorbed. Water can also get past the seals in wheel cylinders and callipers.
Where Does the Water Go?
Once the water is inside the brake system, it is absorbed into the glycol based brake fluid and dispersed throughout the system. If you are going to have water in your brake fluid, you actually want it dispersed throughout the fluid because it minimizes the chance of corrosion caused by localized pockets of water. It also prevents a pocket of water in a calliper boiling, which would occur around 100ºC, much lower than the boiling point of the brake fluid.
What Can be Done to Prevent the Water From Getting in?
So long as you stick with glycol based brake fluid, you can’t stop it, but you can slow it down. To combat the hygroscopic nature of glycol based brake fluids, brake fluid manufacturers add chemicals to the glycol base compounds. Many British vintage cars have metal brake reservoirs fitted with metal caps. To check the fluid the cap must be removed. The switch to transparent brake and clutch fluid reservoirs that occurred in the 1960’s was made in part to eliminate the need to open the reservoir, but there is no easy way to retrofit your vintage car with a clear plastic reservoir. When you check the fluid, do it quickly. The important thing to remember is that although we can slow it down, we cannot stop water from getting into the brake fluid.
At What Point is The Lower Boiling Point a Problem?
Obviously a lower boiling point is not going to be a problem if the brake fluid never gets that hot. That depends on how hot your callipers or wheel cylinders get and on how hard the brakes are applied and how fast the car is going. The brakes are going to get much hotter coming down a long descent on a mountain road than they are motoring through town. Absorbed water reduces the temperature at which gas bubbles begin to form in the brake fluid. When these bubbles form, they turn the virtually incompressible hydraulic fluid into a mixture of gas and liquid which can be compressed considerably. When this happens, the brakes feel “spongy” and the brake pedal travel will increase. It may be necessary to 'pump' the pedal to get the brakes to function.
Water Contamination & Corrosion
The reduction in the boiling point is clearly more of a concern than corrosion, but corrosion will eventually pose a serious problem. First, the corrosion inhibitors have a lifespan. Time and heat act to break down the chemical corrosion inhibitors. As they deteriorate over time, the protection they provide is reduced. Second, the amount of water in the brake fluid will continue to increase. Water contamination of glycol based brake fluids will eventually lead to corrosion of brake pipes, wheel cylinders, callipers, and master cylinders, resulting in pipe leaks, "frozen" cylinder pistons, accelerated seal wear, and the formation of sludge.
Understanding the Corrosion Process
Water contamination has recently been shown to be a contributing factor, but not the sole cause of corrosion. The fundamental principle driving corrosion of iron or steel is simple: The natural progression is from less stable to more stable, and rust produced by the combination of oxygen and iron or steel is more stable than pure iron or steel. Copper also reacts with oxygen for the same basic reason. Corrosion is a complex process, and different metals behave differently. Metals that are used in combination also behave differently. Some metals are inherently more corrosion resistant than others. Consider zinc, iron, and copper. Of the three, copper is the least prone to corrosion, and zinc is the most prone to corrosion. Iron is in the middle.
Dealing with Water Contamination of Glycol Based Brake Fluids
Flush & Re-Fill
It may seem odd, but a quick look in your owner’s manual will give you the answer – regular flush and refill of the brake fluid. The Austin-Healey 100-6 and 3000 Workshop Manuals for example specify brake fluid changes every 18 months or 24,000 miles. As old is the information is, it is still excellent advice today.
Choosing a Glycol Based Fluid Based On Service Interval
It is apparent that the drop in boiling point of DOT 3 brake fluid over time due to water contamination means you must consider changing the fluid every 12 months, regardless of how much you drive. If you live in a dry, arid climate, you can extend that somewhat. Conversely, if you live where it is wet and humid, the 12 month interval might be too long. If you use a DOT 4 fluid, be aware that it actually absorbs water faster than DOT 3 fluids, but the reduction in boiling point is less. With DOT 4, consider changing the fluid every 18 to 24 months. DOT 5.1, consider changing the fluid every 5 years. DOT 6 or Racing Brake Fluid consider changing the fluid every 10 years. These are conservative recommendations, but they do not guarantee that you will eliminate the chance of a brake system vapour-lock related failure because they do not take into account the actual amount of water contamination in your brake system. To find out, you will need to test samples of your brake fluid.
Compared to glycol fluids, silicone has some distinct advantages. They are very stable over wide temperature ranges, and they resist physical and chemical change under severe heat, cold, sheer, oxidation, and other operational conditions that will break down other fluids. They are inert, non-corrosive, non-toxic, and have low volatility. They will not affect paint work. Silicone fluids also have “the lowest viscosity change with temperature of almost any hydraulic fluid.”
Unlike glycol based fluids, silicone fluids are not hygroscopic. Silicone brake fluid will absorb a tiny amount of moisture (on the order of 280 parts per million, or .0028%) and then absorb no more. If we have a brake system with a total volume of 900mls the maximum amount of water absorbed will amount to 0.0252mls. Because water will not mix with silicone fluid, any water that gets into the system will tend to pool in the lowest parts of the system. This resistance to water absorption is a critical difference that makes silicone fluids attractive for cars that are driven seasonally, which makes the longer term issues of corrosion more important that they are with a daily driver.
Silicone fluids have very high dry boiling points – generally around 280°C / 536°F.
Hazard to Paint
Unlike glycol fluids, silicone fluids do not damage paint. This is of particular importance for show-cars where a spill or leak of glycol fluid can have seriously damaging results. A newly rebuilt and scrupulously clean brake system filled with silicone fluid should outlast a system filled with glycol fluid by several times.
Silicone fluids have what appears to be an obvious advantage over glycol based fluids. Given all the trouble caused by water contamination of glycol based brake fluid, silicone fluid has some appeal.
Because of the dissolved air, silicone fluids are up to three times more compressible than glycol based fluids,. This can contribute to a slightly spongy feeling brake pedal, particularly near the higher end of their temperature range but well below the dry boiling point.
Converting from Glycol to Silicone Fluid – The Decision
If you do decide to convert to silicone fluid, it should be done as part of a total brake system overhaul, with freshly rebuilt or new callipers, wheel cylinders and master cylinder. Silicone fluid should not be added to a system which contains even small amounts of glycol fluid or other contaminants. Merely bleeding the system is not enough. However, done well, a "new" system full of silicone fluid will require very little maintenance for years.