Brake Fluid Tech

Tech Services at Moss gets calls every week from people who have questions about brake fluid. Some are concerned because the old familiar brands have changed. Castrol LMA now says “synthetic” on the bottle, while the Lockheed Premium fluid has been replaced by Super DOT 4.

Others have questions about the ever-contentious silicone brake fluid. They have heard or read opposing opinions; some saying it is the greatest brake fluid ever, while others say it is a serious threat to life and limb. In an effort to address the general uneasiness and specific questions, we decided to put together some information that we have collected on the subject.

Fluid Basics
Brake fluid transmits the force of your foot on the pedal to the brake pads and shoes. To do this efficiently, brake fluids must be non-compressible. They must also not boil at the highest operating temperatures encountered; not thicken or freeze at cold temperatures; not corrode or chemically react with any materials in the hydraulic system; and not decompose or form sludge, gum, or varnish at any temperature.

Brake fluids must lubricate internal moving parts, flow easily through small passages, have a long and stable shelf life, and be compatible with other fluids. These properties are specified for brake fluid sold in the U.S. by the Federal Motor Vehicle Safety Standard 116 (FMVSS116), which comes from the Department of Transportation (DOT).

Brake fluids are all synthetic chemicals and are NOT petroleum based. The DOT class system sets standards and testing procedures only; the ingredients used are up to the manufacturer. If it meets the standards, it will qualify as “brake fluid.” Based on a combination of the properties determined by testing, glycol-based brake fluids are labeled DOT 3, 4 or 5.1.

Then there’s DOT 5 fluid, which by weight contains 70 percent diorgano polysiloxane—which we call silicone. Only silicone-based fluids met the DOT 5 standards when they were created. As a result, DOT 5 has come to be synonymous with silicone brake fluid.

When glycol brake fluids with borate esters appeared that met DOT 5 specifications, they introduced them as DOT 5.1 to avoid the confusion of having fluids with two very different bases under the same DOT number. Unfortunately, for many people, the 5.1 implies some connection to DOT 5 and that has created confusion rather than prevented it.

Think of 5.1 as a glycol-based DOT 4 fluid that meets DOT 5 standards. The 5.1 fluids are used primarily in vehicles equipped with ABS brake systems.

Brake fluid absorbs moisture over time, meaning it should be regularly flushed. Don't let today's brake fluid choices confuse you.

Shop Smart
When it comes to picking a brake fluid for your British sports car, do not consider anything that does not meet FMVSS 116. DOT 3, 4 and 5.1 fluids are all hygroscopic, meaning they will absorb water out of the atmosphere. This will lower the boiling point of the fluid.

This lower or “wet” boiling point represents the expected performance of the brake fluid after it has been in the car for one year. For a street car, the wet boiling point is more important than the dry because it is going to be in use for much longer periods.

Both Castrol GT LMA and the Lockheed Super DOT 4 meet the DOT 4 specifications. They are very similar, and the wet boiling point for both is significantly above the standard for DOT 3 fluids.

In addition to lowering the boiling point, the water in the system will corrode the metal components if given enough time. All brake fluids have corrosion inhibitors, but these break down and become less effective over time. Glycol fluids must be completely drained and replaced every 18 to 24 months, regardless of how much you drive your car.

A small word of caution: Glycol fluids will strip the paint off the car if they are spilled or if you have a leak.

Silicone Solution?
Unlike glycol-based fluids, silicone brake fluid will not absorb water from the atmosphere or act like a paint remover. Silicone has very high dry and wet boiling points.

However, it is more compressible because it will absorb more air than a glycol-based fluid. The air is in solution, and should not be confused with air bubbles.

The dissolved air gives a slightly spongy pedal feel which most people cannot detect. It makes silicone fluid a poor choice for racing, but it is considered an appropriate choice for a classic car that is not driven daily. Bleeding systems with silicone takes more time and may have to be repeated because it takes time for the air to work its way out.

Although all fluids meeting DOT specifications must be compatible, mixing them is not recommended. Changing from glycol to silicone fluid is not a trivial undertaking. It is recommended to be done only when the entire system is being overhauled. Even though all brake fluids in the same DOT class must meet the same standards, they will differ slightly. Check with others that have a car like yours before changing fluids.

More Details
Castrol GT LMA  is familiar to most British car owners. The LMA stands for low moisture absorption. The recent addition of the word Synthetic to the label caused some concern, but this fluid has always been synthetic. They added the extra word to the name when they changed the composition to raise the dry boiling point from 446 ºF to 509 ºF. Castrol GT LMA is suitable for use in any British brake system designed for glycol fluid.  220-455, 12 oz., 360ml

Lockheed Super DOT 4  is also a low moisture absorbing glycol-based hydraulic fluid. The dry boiling point is 9º higher than the Castrol, but the wet boiling point is 32º higher than the Castrol.
220-400, 19.9 oz., 500 ml

Cartel Silicone Brake Fluid will not absorb water from the atmosphere, but any water introduced into the system will tend to puddle in the low points. When converting to silicone, completely drain and clean all the lines to make sure that none of the old fluid, water or contaminants remain. Renew all the seals in the system, assembling all components with silicone fluid as a lubricant.
220-410, 1 quart, 846 ml

Any of the three fluids above should perform well in a British brake system. However, seal expanders in these fluids may create problems in some systems where a seal can block an orifice in the master cylinder. Do not change fluids because of this, or any other article you read about brake fluid. Consult with your mechanic and members of your club with the same brakes system you have.
582-505 Gasket, Brake Fluid Reservoir (1.75-inch cap)

For more info on this gasket, visit

By Michael Grant


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