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BRAKE FLUID Types and Opinions - HT-115

At 01:27 PM 5/21/06, Michal Takac in Slovakia wrote:
"Which type of brake fluid is optimized for MGA?"

I have tried for years to avoid this question. If you ask this question on an open forum it will start a religious war where everyone is certain he is right and anyone else is wrong. The bottom line is, use whatever you like. If the system has been recently rebuilt the modern seals should be suitable for any commercial brake fluid.

When the cars were new in the 50's and 60's they used natural rubber seals, and many commercial fluids would dissolve the natural rubber parts. The only prescribed fluid in those days was Girling fluid, commonly referred to as "the red stuff" or crimson fluid. It had a vegetable oil base which was compatible with natural rubber parts. I don't think anyone has produced natural rubber seals since the early 70's, and the crimson fluid has been generally unavailable since the early 90's. Do don't worry about it.

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 commonly available fluids in North America are DOT3, DOT4, and DOT5.

DOT5 fluid is silicone oil (in North America), which is a natural preservative for all kinds of elastomer parts. For a restored classic car, the most endearing feature of silicone fluid is that it will not eat the paint off the body if the master cylinder happens to leak. The most significant deterrent to using it would be the price, as it may cost over $20-USD per quart. But it will last a long time, will save maintenance cost in the long run, and is the only hydraulic fluid I would use in my MGA.

There is recently on the market a DOT-5.1 fluid which is not silicone, but has a glycol base more closely related to DOT4 type, but with a higher boiling point. This is probably expensive, if you can find it, but you have no need to use this in your MGA.

DOT3 is a glycol base fluid which has been used for almost everything American since it was so labeled in the 50's. The crimson fluid held on much longer in Europe. By the time Europe was ready for the change to synthetic rubber seals DOT4 was already available, which was then specified for mid-production MGBs. A little later the specification for MGB was changed to DOT3 fluid, probably as a cost cutting measure.

DOT4 is an uprated version of DOT3, rated for higher temperature. Certain versions of DOT4 fluid are also noted for low moisture absorption (such as Castrol LMA DOT4 fluid). The low moisture absorption characteristic increases the boiling point a little, and also helps to reduce corrosion of the metal parts if the fluid is not changed for a long time. When the crimson fluid became unavailable it was customary to switch to DOT4 and to shun DOT3 fluid. This was probably because most of the European cars which previously used the red stuff had switched to DOT4 for new production cars, and people had an idea that DOT4 would be compatible with the earlier seals (which it is not).

People who are determined to use DOT4 usually cite problems with use of DOT5 fluid (generally imaginary problems). They might say that DOT5 fluid is compressible, which it is, sort of, but the compressibility is so minute as to be negligible. This in itself will not give the soft pedal feel which is so often quoted. They may also say that DOT5 fluid traps tiny air bubbles, which gives the soft pedal feel. The only precaution here is, don't shake the bottle, and pour slowly. It is very unlikely that you will ever have this problem, but if you do happen to get bubbles in the DOT5 fluid, let it set for 12 hours before using it. If you have bubbles in the fluid in your car, let it set for 12 hours and bleed the system again. The DOT5 fluid does not retain air bubbles indefinitely, only for a short period of time, after which the bubbles rise to the top and disperse.

The nay-sayers may also say that DOT5 has the ability to sneak through very small spaces, which may lead to leaks which you may not have with other types of fluid. This is an "old wives tale", generally untrue. If your hydraulic seals leak while using DOT5 fluid the cylinder is about due for a rebuild anyway, and it may leak as well with any other fluid. The last argument against DOT5 is that it may not be quite as good a lubricant as other types of fluid, which could cause cylinder seals to stick and prevent return of the pistons. This is generally also untrue. People may get these ideas when a cylinder may leak or stick immediately after a cylinder rebuild when using DOT5 fluid. But in fact the same thing is just as likely to happen with other types of fluid. This may be a function of slightly rough surface on the cylinder wall after honing.


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 normal operating 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. Here's the technical stuff, in case anyone may be interested.

D.O.T. Minimum Boiling Points (degrees Fahrenheit):
 D.O.T. 3D.O.T. 4D.O.T. 5
Dry401446500
Wet284311356

*This is the required min. specification, but does not reflect actual performance of silicone-based fluids. Since D.O.T. 5 is non-hygroscopic, its actual "wet" boiling point is essentially the same as its dry boiling point. However, a brake system using silicone-based fluid with water present in the system would show an effective boiling point of 212 degrees due to the free water.

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. To further confuse matters, there is now a D.O.T. 5.1 brake fluid which has a diethylene glycol-ester base, with properties similar to D.O.T. 4, but with enhanced performance characteristics. These D.O.T. (Department of Transportation) specifications also indicate minimum boiling points. It is important to note that these D.O.T. specifications are performance specifications, not material specifications; for example, D.O.T. 5 sold in Europe is not silicone-based, as it is in the USA.


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.

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 hygroscopic (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-hygroscopic (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.

A newly rebuilt and scrupulously clean brake system filled with silicone fluid should outlast a system filled with glycol fluid by several times. A "new" system full of silicone fluid will require very little maintenance for years. Even though this may be true, I still recommend flushing fresh fluid through the system every two years like clockwork, to be sure there is no moisture accumulating. When you do get moisture in the system using Silicone fluid, the water will pool as droplets in the lowest parts of the system. Because fluid moves and mixes regularly in the pipes and hoses, the moisture (with silicone fluid) is not likely to affect steel brake lines (at least not from the inside). Water will most likely pool in the slave cylinders, ultimately causing corrosion and pitting of the cylinder walls.

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. Also examine all fluid seals and hoses in the hydraulic system (with replacement as required) every few years. 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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