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The Real Story:
Conventional vs. Silicone Brake Fluid

Today's current Lockheed "Universal", Girling "LMA", and silicone brake fluids are so vastly superior to the old Girling "Green" and "Crimson", and Lockheed "Heavy Duty" fluids originally specified for most of our older British sports cars, that it would make no sense to use the older types today, even if they were still available. The most notable advances have been in raising boiling points, improved compatibility with each other, and reducing moisture absorption.

The main function of brake fluid is to transmit pedal movement 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, 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. 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.

Brake fluids are classified by their chemical type and boiling points. The different chemical bases currently used are polyalkylene glycol ether (commonly called glycol), silicone, and mineral oil. (Of these, mineral oil doesn't concern us, as it is used in very few cars, none of which Moss Motors deals with.) D.O.T. 3 and D.O.T. 4 brake fluids are glycol-based, while silicone-based fluids are classified as D.O.T. 5. These D.O.T. (Department of Transportation) specifications also indicate minimum boiling points.

In the good old days, little good could be said of brake systems. Warnings such as, "as the cups in the master cylinder are pure rubber, it is imperative to use only the recommended fluid. Any other fluid may be dangerous" were common. Such strong concerns that were very valid in the 1950s, are much less so now, even for 1950's vintage cars. The reasons for this lessened worry about our hydraulic systems "turning to goo" if the wrong fluid is used is that: 1.) pure rubber hydraulic seals are no longer made for our cars, and 2.) D.O.T. 3, 4, and 5 brake fluids are safe to mix, and are compatible with the seals now available. While these brake fluids are safe to mix, mixing them is not recommended.

When brakes are applied on a moving car, the kinetic energy of the car is turned into heat. The faster the car is moving and the faster it is stopped, the more heat is produced. Some of this heat soaks into the brake fluid. In the late 1940s, brake fluid with a boiling point of 235° F was considered adequate. By about 1957, the lowest S.A.E. specification was for a minimum boiling point of 302° F for cars with drum brakes.

Disc brakes presented new problems. In stopping faster (and often heavier) cars more quickly, they generated even more heat which had to be dissipated, with an accompanying requirement for brake fluid with even higher minimum boiling points. Improvements in brake lining materials, brake drum and rotor design and metallurgy have also had a similar effect; improvements in braking efficiency require improvements in brake fluids. To handle these higher temperatures, improvements were also made in wheel cylinder and brake caliper seal design and materials.

D.O.T. Minimum Boiling Point Specs are:
Min. Boiling Point (°F)
  D.O.T. 3 D.O.T. 4 D.O.T. 5
Dry 401 446 500
Wet 284 311 356*
*This is the minimum required by this specification, and does not reflect actual performance of silicone-based fluids. Since these fluids are non-hydroscopic, the actual "wet" boiling point is essentially the same as the dry boiling point.

Brake fluids must not be allowed to boil for two reasons:

1) The brakes won't work due to the vapor bubbles being compressible.

2) Physical and chemical properties of the brake fluid may change due to the "lighter" components boiling off. Glycol-based brake fluids in particular, are hydroscopic (moisture absorbing), some more so than others. When water is absorbed, the boiling point is sharply lowered. This occurs because water boils at only 212° F. When brake fluid is mixed with water, the boiling point of the mixture is less than that of the "dry" brake fluid. See chart for D.O.T. minimum boiling point specifications.

Water contamination also leads to corrosion of brake pipes, wheel cylinders, calipers, and master cylinders, resulting in pipe leaks, "frozen" cylinder pistons, accelerated seal wear, and the formation of sludge. Silicone fluids avoid these problems by being non-hydroscopic (not moisture-absorbing), while glycol fluids can absorb as much as 6% water just by being in a "sealed" automotive hydraulic system for a few years. This moisture is generally absorbed from the air. Some moisture even works its way into brake hoses. Most comes from master cylinder cap vents and resultant condensation in the air space above the fluid, and from allowing cans of brake fluid and master cylinders to remain open to the atmosphere for too long. Silicone fluids absorb a tiny amount of moisture (on the order of 280 parts per million, or .0028%) and then absorb no more.

Silicone fluids, in addition to having high boiling points and being non-hydroscopic, do not damage paint as do glycol fluids. This is of particular importance in regard to show cars where a spill or leak of glycol fluid can have seriously ugly results. There are, however, some disadvantages to silicone fluids. They are slightly compressible, particularly near the higher end of their temperature range. While this is of absolutely no consequence for normal street use, this is why silicone fluids are not used in race cars. (Conversely, racing hydraulic fluids should not be used in street cars. This is because, although racing brake fluids have high dry boiling points, most are highly hydroscopic, and have relatively very low wet boiling points. They would probably work extremely well if you were to change the fluid every week or so.) Because air bubbles do not regularly dissipate in silicone brake fluid, special care must be used to prevent them from forming during pouring and bleeding operations. The best way to bleed a silicone fluid system is with an Eezibleed (Moss #386-860), or Visibleed (Moss #386-885) Kit. Lacking that, bleed with slow pedal strokes, avoiding "pumping" the pedal. It may be necessary to bleed the system again in a day or so if there were any air bubbles which wouldn't bleed out the first time.

A newly rebuilt and scrupulously clean brake system filled with silicone fluid should outlast a system filled with glycol fluid by several times. There is little advantage in adding silicone fluid to a system which contains even small amounts of contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge which will not bleed out. Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge, into slugs, instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems which had been used with glycol fluid. A "new" system full of silicone fluid will require very little maintenance for years.

Old dirty moisture-laden brake fluid is hazardous; it can't be relied upon to stop your car reliably. It is a little known fact that glycol brake fluids must be changed regularly, much as engine oil must be changed. The Austin-Healey 100-6 and 3000 Workshop Manuals specify brake fluid changes every 18 months or 24,000 miles (whichever comes sooner), and examination of all fluid seals and hoses in the hydraulic system, with replacement as required, every 3 years or 40,000 miles. Other manufacturers had similar recommendations. While silicone fluid change intervals may be safely extended, do not overlook periodic checks, especially of hoses. Please take care of your brake system for your own and other's safety.

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