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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. 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.

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 were very valid in the 1950s, 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.

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.

 

D.O.T. Minimum Boiling Points (degrees Fahrenheit):
D.O.T. 3 D.O.T. 4 D.O.T. 5
Dry 401 446 500
Wet 284 311 356

*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 silicon-based fluid with water present in the system would show an effective boiling point of 212 degrees due to the free water.

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.

Silicone fluids, in addition to having high boiling points and being non-hygroscopic, 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 hygroscopic, 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 Kit (Moss #386-860). 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.

By Michael Grant, Moss Technical Services

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'Conventional vs Silicone Brake Fluid' have 18 comments

  1. May 15, 2012 @ 4:15 am Paul Bingham

    I have silicone in a TR3 and works well you explained the fluids well thanks

    Reply

  2. May 15, 2012 @ 8:39 am Michael Newhouse

    I’ve been running silicone fluid in clutch and brake hydrolic system in my 67 MGB for over 20 years and have not required new rear brake cylinders and only one caliper replacement (both sides), one master cylinder and no clutch master or slave cylinder replacements. Only caveat: The brake light switches have regularly worn out after 3 or 4 months until I recently installed a SW-32 LOW PRESSURE HYDRAULIC BRAKE SWITCH from Ron Francis Wiring and it appears that problem is solved.

    You must let anybody working on your car know you are running silicone and put a notice on the master cylinders.

    Reply

  3. May 16, 2012 @ 7:29 am John Burton

    I to have been running Silicone dot 5 fluid in a TR3 for 11 years prior to selling it in 2008, and in my 100-4 Healey since 2002. Both had rebuilt systems and the only issue I have had is also the failure of the brake light switiches. I fitted a TR6 switch under the brake peddle to resolve the issue. I use Castrol LMA in all my customers cars.

    Reply

  4. May 18, 2012 @ 7:52 pm John Borchardt

    I tried silicone brake fluid in my ’76 TR6 and within 2 years and 1000 miles I had leakage from my rear pistons. I replaced the rear pistons, purged the system with DOT4 and have no further problems, religiously changing the fluid every 3 years. No more DOT 5 for me.

    Reply

  5. May 20, 2012 @ 11:50 am Mike Daniels

    Thanks to Michael Grant for an excellent treatise on brake fluids, I hope to use the info as the basis for a
    “Tech Session” on maintaining MG brakes with our local MG Club.
    I too am a silicone (DOT 5) fan, I first used it in 1975 in an AH3000 ground up resto (sold in 1998), and since 1980 when I bought my ’66 MGB. The ‘B still has the original master cylinder and front callipers both with new seals and new rear wheel cylinders fitted for the complete brake system overhaul in 1980.
    10 years ago I stripped the brake system down to check overall condition. The seal wear was minimal with no rubber deterioration and there was no trace of corrosion on the pistons or bores. I did however, notice a tiny bubble of water in each calliper (as Michael predicts) but no corrosion around it.
    The fluid has been replaced every 5 years and the car has been driven, summer only, 80,000 miles since 1980, 130,000 miles from new. It has a 1991cc 125 bhp engine and is driven fast and hard.
    Also has Goodridge flex hoses and CuNi brake lines.
    After several failures of hydraulic brake light switches, I gave up and converted to the later style mechanical switch actuated by the brake pedal. I think the failure is due to silicone fluid molecules passing through the switch diaphragm and getting onto the contacts, arcing at the contacts causes the silicone to coat them with an insulating film.

    Reply

  6. May 25, 2012 @ 11:55 am Frank Stabler

    The word is hygroscopic, not hydroscopic.

    Reply

  7. May 29, 2012 @ 4:06 am Fred Schmidt

    When I restored my 1966 Triumph TR4A Daytona 24HR race car in 1992, I put silicone in a completely new system. Everything was new down to the steel lines. The brake master cylinder has failed 2 times since then. The rubber seals were like jelly and came apart in my hands. This car is rarely driven and in my garage in Florida. My other 2 Triumphs, a 1966 TR4A and a 1972 TR6 are driven regularly and have had the same master cylinder break down regarding the seals. Not sure what is happening here but now there is a warning on the box saying ” use of silicone fluid voids the warranty”. No more silicone for me. Currently, Castrol GTLMA is my preferred brand.

    Reply

  8. May 29, 2012 @ 5:51 am David Holmes

    This topic is the source of a seemingly endless debate. My experience is that Silicone, because it is non-hygroscopic, allows moisture to collect in the bottom of the Master cylinder rather than in suspension within the fluid, which causes pitting and subsequent failure. In my case the clutch side failed to seal and allowed fluid to squeeze past the seal whilst the pedal was depressed, disengaging the clutch and causing a rapid rise in my heart rate. This was in a new repro Master Cylinder with less than 3000 miles clocked. The curious part of this debate is that there are as many owners with similar experiences as mine as there are owners who swear by Silicone. I was greatly disappointed that I could not use Silicone because of its non-paint removing properties in my restored MGA and was convinced to return to the glycol based dot 4 for safety reasons. There are warnings published by rubber seal manufacturers as well. I had my MC re-sleeved with stainless after the failure and I wonder if I could have stayed with Silicone rather than completely replacing all cylinders and purging all lines which was quite expensive. (after another 3000 miles with dot 4 I have had no problems. *knocking on wood*)

    Reply

  9. May 29, 2012 @ 3:31 pm Randolph Watson

    My first experience with silicone brake fluid was in 1980 with my 1963 MGB. After overhaul of the engine, transmission, clutch and brakes at 124,000 miles, I managed to spill the standard glycol type fluid in the area of the master cylinders which resulted in the very undesireable removal of the paint in that area. I decided to try silicone fluid which does not affect the paint.. Since then I have had no problems at all with the hydraulic systems. The car now has more than 183,000 plus miles on it.

    In 1993 I bought a 1973 MGB GT which had not been driven for ten years. I completed a a total restoration the car in 2003 with all mechanicals, engine, transmission, clutch, drive train and brakes completely overhauled/rebuilt. The body was taken down to the bare shell and everthing removed from the engine bay including all fuel and hydraulic lines. In view of my positive experience with silicone fluid in the 63 B, I installed silicone fluid in both the brake and clutch systems and have had no probglems traceable to silicone fluid. And, since both hydraulic reservoirs can be difficult to get to I have had at least one occurrence when inadvertant spillage of silicone fluid saved my paint (and my bacon).

    I also have a 1947 Ford which had silicone fluid in the original brake system when I aquired it in 2006. I have had zero problems with that system in the 4,000, or so, miles I’ve driven it. So, am I a fan of silicone hydraulic fluid??? YOU BET!

    Reply

  10. May 29, 2012 @ 3:59 pm Bob Rodine

    I have a ’74 MGB. It has not been driven since 1986. I just installed new brake and clutch master cylinders. I have not yet loaded the system with fluid, but am inclined to go with DOT 5. I originally planned to clear the old fluid from the system by bleeding it, but after reading the comments and article am inclined to believe that the lines should be flushed. Is denatured alcohol the best fluid to flush the lines with, and what sort of resevoir does one use to hold the flushing agent the top end of the lines? Is the #386-860 EZBleed the best way to draw the flushing fluid through the system?

    Thank you much.

    Bob Rodine

    Reply

    • May 30, 2012 @ 2:06 pm Tamara Cribley

      Hello Bob.

      Moss Tech Services has a bit of info for you:

      You can put a small amount of DOT 5 in each brake line and then follow up with compressed air. That will clear them.

      Most of the available information is allegorical, but conventional wisdom says all rubber parts need to be replaced before the conversion. That means seals in the master cylinders, and seals in the wheel and slave cylinders. It’s assumed you will replace the three rubber flex hoses too. Anything rubber that was used with Dot 3 or DOT 4 should be replaced.

      386-860 Eazileed works fine.
      Note: 1974 was a transition year. The chrome bumper car was the early version. The rubber bumper car was the 74 1/2 version. See http://www.mossmotors.com/graphics/products/PDF/386-860.pdf to know which cap to buy.

      Finally. DOT 5 has a propensity to develop micro-bubbles while bleeding. These settle out in a few to twenty-four hours. The problem is if you collect fluid at your wheel cylinders and put it back into the bleeder, you will collect micro-bubbles in the system. You will tear your hair out trying to figure out why you have spongy pedal. As you draw off fluid at the wheel cylinders you can put it into a jar to stand over night. Do not put it immediately back into the system.

      Reply

  11. June 1, 2012 @ 11:05 am Bill Bauder

    Have been using silicone in my brake system on the Allard for 10 years without any problems. One thing it really does well is it keeps the aluminum wheel cylinders spotless inside. You might mention that silicone has a tendency to creep through threaded joints and that it is advisable to use a soft thread sealer on all threaded joints.

    Reply

  12. June 1, 2012 @ 11:35 am John Henriksen

    I’ve worked on British cars for years & have done many brake jobs. I’ve found that silicone fluid is not superior to regular fluid except that if spilled on paint, it won’t ruin it. I’ve found that it takes a harder pedal to stop with silicone. Also, Apple Hydraulics who are expert brake system re-builders state in their directions that use of silicone fluid will void their warranty. Not sure, but I think there may be a problem with silicone and the seals. I stick with Castrol LMA & am very careful to avoid spills.

    Reply

  13. June 6, 2012 @ 3:45 am Dave Quinn

    Michael, thanks for an excellent technical article on brake fluids and silicone fluid in particular. I’ve had 5 trouble free years with silicone in my current MGA and 10+ trouble free years in the MGA that came before it. As you clearly point out, you have to start with a completely ‘clean’ system and ‘new’ rubber parts. Done properly, it works great. Of course, the age old disagreement will continue.

    Over that period of time I have had a brake light switch failure but based on fellow owners I don’t think it’s any more common than with MGA’s using DOT3. One tip I heard, but have not tried, was use a eye dropper to remove any potential air pocket within the switch itself when installing a new one.

    Reply

  14. November 24, 2015 @ 7:48 pm David Younkin

    I have a 1972 Corvette and a 1972 Jaguar xke. I bought them in the late 70s. I put silicone brake fluid in both of them as soon as I got them. I don’t drive them in wet or snow. I have not touched the brake systems after I put the silicone brake fluid in.

    Reply

  15. May 25, 2019 @ 6:25 pm Andrew McDougall

    I live in Melbourne, Australia. I have a 1940 Speed 25 Alvis Saloon which is undergoing a full mechanical and body restoration. The mechanical brakes on this car were replaced by a duel hydraulic system in 1950. During my restoration I have had all the wheel and master cylinders fitted with stainless steel sleeves and new seals, all steel lines and rubber brake hoses replaced and new soft copper washes fitted in all the unions. Due to planned occasional use once the car is back on the road I decided to use DOT 5 Silicone fluid so as to eliminate the hygroscopic and corrosion effect. However since filling the system with silicone fluid and bleeding the system I have been plagued with leakage where the steel lines screw into the brass unions, the unions with the soft copper washes have leaked where they attach to the master and wheel cylinders and under gravity the the silicone fluid is leaking past the rubber seals in the wheel and master cylinders and over time draining the reservoir of fluid. I would like to persist with the silicone fluid with its non gyroscopic effects and suitability for potential trouble free braking in an occasionally used vehicle, however what do I have to do to the system in order to keep the fluid in and stop leakage?

    I will appreciate someone from Moss or some other knowledgeable person getting back to me as this is proving to be a serious and frustrating issue for me!

    Reply

  16. July 4, 2019 @ 10:04 am Chris Rider

    Restored a ’66 Vette coupe in the ’90’s. Put silicone brake fluid in ta that time and never a problem in tow plus decades.

    Reply


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