Using silicone brake fluid in a vintage truck creates a love/hate relationship. A lot of misinformation is circulated in both hobby literature and via “conventional wisdom” about the practice.

The Society of Automotive Engineers Standard J1703 (SAE J1703) and Federal Motor Vehicle Safety Standard 116 (FM VSS 116) spell out in exquisite detail how to test brake components and fluids and how to measure their compatibility, among many other specifications. Here are my recommendations for building a trouble-free brake system based on real engineering.

Silicone brake fluid is also known as DOT 5 fluid (Department of Transportation). Our old trucks were originally filled with DOT 3 fluid. It is chemically based on triethylene glycol monomethyl ether (similar to antifreeze) but has a longer chain with a higher boiling point. DOT 4 fluid is chemically similar to DOT 3 but has a higher boiling point.

The big differences are:

• Silicone, when sucked through a leaking vacuum booster into an engine, burns to form silica sand and quickly wears down an engine's internal parts. Glycol burns harmlessly.
• Silicone affects rubber brake components' “elastomers” differently than glycol and may not be compatible.
• Silicone will not mix with water; glycol will.
• Silicone will not lift paint; glycol will.
• Silicone contamination in your shop will cause fisheyes in future paint jobs; glycol will not.

Vacuum Boosters Take a sample of silicone (I used gasket sealer years ago) and burn it with an acetylene torch. It oxidizes to a fine white powder. This is sand. If you have a vacuum-brake booster, some leakage into the engine is possible, so silicone brake fluid is not for you. Use glycol DOT 3 fluid to avoid an engine disaster. Any potential good from silicone is far outweighed by massive engine problems if it gets past the booster seals.
Use glycol DOT 3 fluid if you have a vacuum booster.

Elastomer Compatibility
Brake manufacturers depend on their elastomers to swell up to 5 percent to properly seal. With glycol fluid this happens consistently, as everything was engineered for glycol at the OEM level, when the new truck was manufactured Also, new cups in a new cylinder start with an interference fit of up to 0.080 inch.

The historic problem with silicone fluid was inconsistent swelling with elastomers. Sometimes the brake system started leaking a month after changing over to silicone. Upon disassembly and inspection, the rubber parts were mushroomed out and as soft and sticky as melting ice cream. Finding out the ingredients in silicone brake fluid is impossible, as they are proprietary information or “trade secrets.”

Three different types of rubber are commonly used in brake systems. If swelling agents have been added to silicone fluid, they do not act consistently with the various rubber components. EPDM and SBR rubber are found in hydraulic system seals. Coinponents made many years ago may contain natural rubber. About 20 years ago, SBR, which could take more heat than natural rubber, was the material of choice. Now EPDM is the material of choice as it has even higher heat resistance. Hoses made from neoprene may have a more resistant inner liner, such as SBR, in contact with brake fluid.

The question is how to predict if and how much seals will swell after installation in a brake system.

SAE J1703 has a test for brake fluid compatibility with standardized 1-1/8-inch SBR brake cups. This standard is primarily for glycol-type fluid. According to sections 4.12.2 and 5.12.2, standard new cups are immersed in the test brake fluid for 70 hours at 120 degrees Celsius. The cups “shall show no increase in hardness, shall not decrease in hardness by more than 15 points (International Rubber Hardness Degrees) and shall show no disintegration as evidenced by blisters or sloughing indicated by carbon black separation on the surface of the rubber cup. The increase in the diameter of the base of the cups shall not be less than 0.006 inch, nor more than 0.055 inch.”

FMVSS 116 has a similar test in section 6.12, which applies to glycol and silicone fluids. “The effects of a brake fluid in swelling, softening and otherwise affecting standard SBR wheel cylinder cups shall be evaluated by the following procedure: Four standard SAE SBR WC [wheel cylinder] cups are measured and their hardnesses determined. The cups, two to ajar, are immersed in the test brake fluid. One jar is heated for 70 hours at 70 degrees Celsius, and the other for 70 hours at 120 degrees Celsius. Afterwards, the cups are washed, examined for disintegration, remeasured, and their hardness redetermined.” Passing limits are the same as for SAE JI7O3. So, if brake fluid—either glycol or silicone—meets this standard, it will swell standard cups less than 5 percent.

How do we know we are purchasing brake fluid and elastomers functionally similar to “standard SBR cups”? About five years ago, EIS brake company certified that its master cylinders, wheel cylinders, and kits were compatible with its brand of silicone brake fluid. Now the company is out of business. No, I don't see any ironic cause and effect here.

Wagner and Raybestos are the two big brake manufacturers now. I asked Wagner if it had any recommendations about silicone fluid. The Wagner technical department emphasized that it is a supplier of OEM replacement parts so its elastomers should be the same composition as the original equipment.

Company representatives were evasive when asked if they could update 50-year-old specifications to improve their products. Based on the company's OEM commitment, it could not recommend silicone brake fluid in a system designed for glycol fluid. Since no original truck (or car) manufacturers use silicone, they do not test their products with silicone fluid.

My next call went to Raybestos. “Our brake cups, seals, and boots, whether EPDM or SBR elastomers, are compatible with brake fluids specified in FMVSS 116—DOT 3, DOT 4, or DOT 5,” according to Kenneth Kopp in Product Engineering. Remember these are short-term tests that will be indicators, but not a guarantee of compatibility. Also note that the check valve in a master cylinder could be incompatible but still pass the test.

It looks like we've come closer to solving the mystery. Buy brake fluid and rubber parts that are certified to comply with FMVSS 116. Raybestos is a safe brand to use with either type fluid. Raybestos also sells silicone brake fluid.

Read the Brake Fluid Label
FMVSS 116 has a packaging and labeling requirement. According to section 5.2, there must be a tamper-proof inner seal that will be destroyed when the container closure is initially opened. Without hiring a lab to run chemical tests, there is a good indication that the brake fluid meets specifications if all the specified information is on the label and the bottle has an inner seal.

FMVSS 116 packaging and labeling requirements state the brake-fluid label must contain:

(a) certification that the brake fluid conforms to FMVSS 116
(b) the name of the packager of the brake fluid, which maybe in code form
(c) the name and complete mailing address of the distributor
(d) a serial number identifying the packaged lot and date of packaging
(e) designation of the contents as “DOT__ Motor Vehicle Brake Fluid” (The blank will indicate DOT 3, DOT 4, DOT 5 silicone base, or DOT 5.1 non-silicone base, as applicable.)
(f) the minimum wet boiling point in Fahrenheit of the DOT brake fluid in the container
(g) the following safety warnings in capital and lower case letters as indicated:

(1) FOLLOW VEHICLE MANUFACTURER'S RECOMMENDATIONS WHEN ADDING BRAKE FLUID.
(2) KEEP BRAKE FLUID CLEAN AND DRY. Contamination with dirt, water, petroleum products or other materials may result in brake failure or costly repairs.
(3) STORE BRAKE FLUID ONLY IN ITS ORIGINAL CONTAINER. KEEP CONTAINER CLEAN AND TIGHTLY CLOSED TO PREVENT ABSORPTION OF MOISTURE.
(4) CAUTION: DO NOT REFILL CONTAINER AND DO NOT USE FOR OTHER LIQUIDS.

Silicone Contamination
A big advantage of silicone brake fluid is that it does not soften and blister paint the way glycol tluid does. Harley Davidson uses silicone as the factory fill. Its motivation could be to prevent paint failures from spilled fluid. If you splash any glycol fluid on paint, wash it immediately with a wet cloth. Otherwise, it acts like paint remover. Plan a clean-up strategy prior to using glycol fluid.

Silicone, however, even in minute amounts, will contaminate a new paint job. Paint will not wet a silicone-contaminated surface, and a defect called “fisheyes" results, in which little craters appear shortly after paint is applied. If you notice this fast enough on the first spray-gun pass, you can add fisheye eliminator to the paint mix, if it is compatible with your paint system, and continue spraying. The paint may flow out evenly.

Once silicone has dripped on the floor, it seems months or even years later, it can be kicked up-by an air hose dragged over the spot or by compressed air that happens to hit it- and land on previously clean metal. Compare it to recent anthrax contamination in government buildings and how difficult it is to remove every last trace of a substance.

Conscientious painters use a solvent wipe over sheet metal between each preparation step. My recommendation is to add and bleed silicone brake fluid far from bodywork areas. I do it outdoors away from my shop whenever possible and use silicone lubricant spray outdoors. I also stay away from Armorall rubber dressing, as that is sure to contaminate a paint shop with silicone in short order.

Recommendations and Hints
If you live in a dry climate or keep your truck in a dry garage, you don't have to be concerned with your brake fluid absorbing moisture. You should have many years of reliable stopping with glycol fluid.

The moisture factor gets to be significant if your truck stays in a moist environment. If you live in the “rust belt,” silicone fluid could be beneficial in keeping out corrosion.

Light-truck manufacturers do not recommend flushing brake fluid on a regular basis. Some say flushing older high-mileage systems may dislodge corrosion particles in the lines and push them up to rubber cups where they will do damage. This is a trade-off. If you use glycol fluid, you could gently flush every few years. 

Don't press the brake pedal to the floor during bleeding as that may also dislodge crud.

Elastomers should be fresh. SAB test instructions say to use cups within three years of manufacturc. This precludes using flea market parts without careful inspection. Bend NOS cups and inspect for cracks. If the box looks like an antique, count on replacing elastomers.

Brake fluid should also be fresh. Don't buy more than you will use in a short time. (I always buy from a British specialist when working on Land Rovers, purchase brake fluid from the same British supplier and give the leftovers to the new truck owner. British specialists are aware of the special needs of their brakes.)

A simple way to test for fluid type is to put a few drops in a plastic-foam coffee cup. Glycol dissolves the cup, but silicone does not. Glycol also dissolves in water but silicone in water forms two distinct phases.

One concern with silicone fluid is that it may not lubricate pistons as well as glycol. SAE has a test for that, which I will discuss another time. There are also tests for wet and dry boiling points, high- and low-temperature stability, chemical stability, and corrosion. They are all good points to consider.

Points to Keep in Mind

• Don't open a master-cylinder cover when it is raining.
• Don't let dirt fall in when you do open a master cylinder.
• Keep petroleum products away from all rubber components. The engineer at Raybestos said a large percentage of brake failures are due to inadvertent petroleum contamination, and just a small amount of contamination will ruin a brake system.
• EPDM rubber is less tolerant to gasoline and oil than other elastomers.
• Keep the work area clean during brake work. Good housekeeping is important if you want the brake job to last.
• If you are switching to silicone, hobby literature gives accurate instructions. Dow Corning circulates good instructions. Start with all new components.
• Don't flush a connected system with alcohol as you can't get it all out.

Next time, I'll discuss more tests specified by the brake standards. If I made any errors, let me know and let's get them corrected.