Girling vs. Silicone Brake Fluid

By Gary Anderson

Originally published in the Austin Healey Magazine, March 1990. Updated October 2007.


Over the past year I received several technical pieces touting the advantages of silicone brake fluid, and several equally definite pieces arguing that one should never use anything except Dunlop/ Girling brake fluid in a Healey. While these pieces were long on opinion, they tended to be short on technical rationale. Since I had no basis to believe one side or the other, I decided not to publish any of them. Last month I came across the following item in the "Technical Correspondence" column of Road & Track. Since it provides some definitive information, I am passing it on. The conclusion: Newer is not necessarily better.


"A number of readers have written with questions regarding the newer silicone brake fluid versus the long-established glycol-based kinds. To sum it up, silicone fluid's main advantage is its inherently non-corrosive nature. And as any of you who have spilled brake fluid know, glycol-based fluids can be highly corrosive.


[Nevertheless] there seems to be a multitude of reasons to stay with the type of brake fluid originally recommended for the car.


Silicone and glycol fluids are entirely different and this is where the first problem arises.

Silicones have about three times the amount of dissolved air as glycol fluids (approximately

fifteen percent air in silicone versus five percent in glycol). Because of the compressible nature of air, brake systems using almost all their pedal travel with glycol fluids may suddenly be inadequate with the silicone fluids, bottoming out before sufficient system pressure is reached.


The second problem comes from the three different types of rubber used in the brake system. There is EPDM rubber, SBR rubber, and neoprene. Neoprene is used only for brake hoses, while the seals in the system may be composed of the other two types. We are told that most modern brake systems use some EPDM seals and most rubber brake hose is now composed of EPDM rubber, although more than 90 percent of drum brakes still use SBR seals.


Glycol fluid affects all three types of rubber uniformly, causing a swell rate of

five to fifteen percent. This swelling action is necessary for the seals to withstand the immense pressures to which they are subjected. Silicone (by itself] doesn't cause rubber to swell, so a rubber-swell additive is incorporated into silicone brake fluid. Unfortunately current additives are compatible only with the newer EPDM rubber. SBR rubber, when subjected to silicone fluids, will swell too much, becoming too soft for adequate sealing.


Now what happens if the fluids are mixed, for example, by using existing brake lines when doing a restoration, but installing silicone instead of the original glycol-based fluid.

Remember the rubber-swell additives used with silicone? Glycol fluids will cause the swell additive to separate from the silicone fluid. Additionally, silicone fluid will cause the anti-corrosion additives used in glycol fluids to separate. In other words, both fluids will lose the additives that allow them to work."


An additional note should be added here that the R&T technical editor didn’t touch on: Glycol tends to be “hygroscopic” which means it absorbs water from the atmosphere. Over a period of time, the water in solution in the brake fluid will reduce the boiling point of the brake fluid, and significantly reduce the effectiveness of the brake fluid, especially in heavy use. Also, the water in solution will lead to corrosion in the brake system (and in the clutch system if you have a hydraulic clutch)which can lead to the brakes and clutch freezing up from rust if the car has sat for a year or more inside, or as short as six months if stored outside.


I have one Healey where the complete brake system was replaced, and it has all worked fine with silicone fluid for fifteen years. This is only anecdotal evidence, but over the years I’ve talked to at least five curator of major automotive collections, including the Blackhawk Collection in Danville, California, the Petersen Museum in Los Angeles, and the San Diego Automobile Museum. Without exception, every curator indicated that in their inhouse restorations, they always substitute silicone for glycol, because cars in their collection will often sit for several months between use, and they don’t want the brake systems (and clutch systems in most of our British cars) to collect water and rust up. In addition, of course, they’re concerned that the expensive finishes of their cars not be endangered by glycol brake fluid.


So what is the bottom line? The most obvious conclusion is that if you are not replacing your entire brake system, you may be better off staying with glycol-based fluid. However,

if you are replacing all rubber parts with modern rubber and you are replacing the lines so that you can be sure that no glycol fluid is left anywhere in the system, you can use the silicone fluid to get the benefit of its non-corrosive nature, and its greater longevity under intermittent use and long storage conditions. But don't be surprised if your brake pedal travels a little more.


If you do choose to retain glycol, then be sure that you replace it every two to three years, and more often if the car is stored outside and isn’t used regularly. Also, be sure to specify Dot 4 brake fluid, which is more compatible with Girling systems. Silicone brake fluid doesn’t need to be replaced as often, but changing the oil every five years wouldn’t be excessive.


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