The skinny on silicone brake fluid

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mikenixon
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The skinny on silicone brake fluid

#1

Post by mikenixon »

It's a mystery that silicone brake fluid is so controversial still, after all these years. There is a lot of emotionalism and misinformation on the web concerning it, but also some excellent and objective firsthand reports and documentation. Let's address this calmly and examine both glycol and silicone fluid's pros and cons.

But first, we need to get something out of the way. The correct way to speak of brake fluid is by their types, not their DoT (Dept. of Transportation) numbers. Omitting the mineral oil used in bicycles and whatnot, there are just two major brake fluid types: glycol, whose base is related to engine coolant; and silicone, whose composition is akin to the major ingredient in many car "waxes". Glycol is the older fluid, silicone the relative newcomer. In the U.S., brake fluid carries a DoT classification that continues to be confusing to many. However, in short, DoT numbers are all about moisture. They communicate the usability of the fluid after it has assimilated water, by rating its heat resistance before and after water absorption; why most fluids have two numbers, "wet" and "dry". The fluid's ability to resist heat lessens as more water is absorbed; it can overheat easier and begin to be compressible, which means the brakes can fail. This is more a problem in cars than in bikes, but the hydraulic clutch fluid in some bikes (such as the 1200 and later), being attached to the engine, have been observed to fail from heat also.

Many folks are unaware of the silliness of the DoT classification system. Here are just a few examples. First, it is useful only in the U.S. In the states we have four current classifications, but other countries recognize only three. This makes comparison troublesome. Second, the "boiling" temps are not consistent from fluid manufacturer to manufacturer; they can vary quite a bit. Third, though it's vitally important to monitor your fluid's water content, the DoT system merely hints at this need without telling you what to do about it. That would at least lend practicality. Fourth, the DoT system has mislead consumers into thinking that the classification number is a kind of descriptive labeling useful in comparing and purchasing fluids, which of course it is not. Fifth, the DoT system is basically irrelevant in the case of silicone fluid which neither attracts nor assimilates water. Sixth, brake fluid classifications cross fluid types. That is, both glycol and silicone fluids have been assigned the same number. And there's more. But with these real issues, it makes little sense to speak of brake fluid by the DoT numbers 3, 4, 5 and 5.1. It's far more helpful and logical to focus on the fluid's composition. The numbers communicate little that is useful; the fluid *types* tell us everything.

And what the fluid types tell us is what we can expect from each. Their characteristics. Glycol fluid is hygroscopic, meaning it attracts moisture right out of the air, and just as importantly, after attracting it, absorbs it, dispersing it evenly throughout the fluid. It gets this moisture of course through the breather vent in the master cylinder, and also through the brake system's rubber hoses, through which air enters, and thus moisture. For seeming millennia vehicle manufacturers have labored to convince us hygroscopicity is a good thing. They say, better that the moisture is evenly dispersed throughout the brake system than it collect in pockets, because that way the moisture never evaporates to leave compressible air in the system. But this has proven to be a doubtful advantage. At least in pockets it would have opportunity to be cooked out by brake use. Absorbed it can't, and so it continues to accumulate, accelerated by glycol fluid's natural moisture-magnet property, until it ruins the fluid, eventually turning it into mud, and worse, starts to corrode the system's metal parts. Glycol fluid is also fairly caustic, meaning it strips paint and chemically reacts with certain types of plastic, deforming and cracking them, especially that in motorcycle bodywork. Glycol fluid is fairly easy to clean up when spilled. Glycol fluid has high lubricity, encouraging smooth working of brake pistons. Glycol has a wonderful inherent ability to purge air, making it easy even for the do-it-yourselfer to bleed because the fluid resists aeration. Glycol fluid is cheap and readily available. And glycol fluid offers a great visual for when it is absorbing moisture: its color gradually darkens.

Silicone brake fluid on the other hand has very different properties. Commissioned originally by the U.S. Army as part of a maintenance-reducing program and said to have been developed primarily by Dow Corning, it neither attracts nor assimilates moisture. The little water this moisture-neutral fluid does accumulate, less than a tenth of glycol's, by the way, is isolated from the fluid. It does not mix with it. It localizes it. Silicone brake fluid is not caustic. Spilled silicone fluid is more difficult to clean up; that is, it's not easily broken down with a cleaning solvent. Silicone fluid aerates easily and thus requires special technique for bleeding, perhaps its biggest difference. Silicone fluid doesn't have or need glycol fluid's visual mechanism for tracking moisture pollution; i.e. it doesn't darken. Silicone brake fluid is more expensive and not as plentiful. And absent from silicone brake fluid is any natural lubricity, making it hugely incompatible with the mechanical valving in antilock braking systems, and thus destined to become even harder to find in this ABS age.

The controversy surrounding silicone brake fluid revolves around two oft-repeated claims: that silicone brake fluid deteriorates brake system rubber seals, and that it results in reduced brake lever feel.
First, brake system rubber parts. Rubber brake parts incompatibility may be the longest surviving silicone fluid boogieman. But there is no factual support. When using high quality OEM rebuild kits, incompatibility is simply a myth. Plain fact. As for aftermarket kits, as is typical with the aftermarket, some work with silicone, some don't. And even some parts within a given kit work while others don't, which by the way is true also with glycol fluid. This is the way of aftermarket rebuild parts. Avoid them.

Brake system feel. This is the other claim. The misconception persists that silicone fluid is noticeably more compressible. However, in my experience such complaints are invariably due to improper installation and bleeding. There is zero difference in brake function or feel between glycol and silicone brake fluids properly installed and in street use. But doesn't silicone brake fluid aerate easily? Yes. It is instructive to look on a bottle of Harley-Davidson silicone brake fluid, where you will find the warning to "allow the bottle to rest on the workbench for an hour before using it." Yes, silicone fluid aerates quite easily. Thus bleeding silicone can be a challenge. You can't manhandle it. And you can't use the traditional lever-pump-then-valve-open method of bleeding. It just won't work. Those with experience with silicone fluid use either pressure or vacuum bleeders. If there is a "trick" to using silicone brake fluid, this is it.

Consider silicone brake fluid a viable alternative, even a boon, despite the Internet's uninformed rhetoric. Silicone fluid eliminates the need to regularly rebuild your infrequently-ridden vintage bike brake cylinders. This is why the stuff was invented, and it continues to be silicone fluid's reason for being. It just doesn't eat up your aluminum brake parts the way glycol fluid does. Silicone brake fluid is also friendly to paint and plastic and is even used by detailers to clean and "polish" paint finishes. Far from a bad thing, silicone brake fluid is one of the best things to happen to vintage motorbikes.
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Re: The skinny on silicone brake fluid

#2

Post by Seb Astian »

A very informative read. Thanks for putting this together!

How would silicone fluid react to leftover glycol fluid in the system? What would be the best way to flush the system, or is it necessary to replace hoses?
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Re: The skinny on silicone brake fluid

#3

Post by Track T 2411 »

Seb Astian wrote:A very informative read. Thanks for putting this together!

How would silicone fluid react to leftover glycol fluid in the system? What would be the best way to flush the system, or is it necessary to replace hoses?
Exactly my questions, and in addition, the rubber parts of the calipers and masters...
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Re: The skinny on silicone brake fluid

#4

Post by tomk1960 »

I've been using DOT 5 silicone brake fluid in all my restorations for years and swear by it. I've heard that if it mixes with DOT3 or DOT4, it will gel and cause problems. In all but one of my restorations, I completely overhauled the calipers, master cylinders, and installed new SS braided brake lines. In the one exception where I overhauled everything but used the original lines, I blew out the lines with BraKleen followed by lots of compressed air. I had no issues whatsoever.

If you decide to switch to DOT5, you will have to disassemble and clean the calipers and masters. You could definitely wash the internals in warm water with a little dish detergent, but why skimp? Just rebuild them with new internals, install new lines, and then you're done for a VERY long time.

There are proponents to DOT5 that will vehemently state that it will destroy seals, etc. Not true and I have experience to prove it. I love it and will never use the other horrible brake fluids again.

And DOT5 isn't as hard to bleed as one might think. Here's proof:https://www.youtube.com/watch?v=m4wlSQDsEHw
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Re: The skinny on silicone brake fluid

#5

Post by mikenixon »

Seb Astian wrote:A very informative read. Thanks for putting this together!

How would silicone fluid react to leftover glycol fluid in the system? What would be the best way to flush the system, or is it necessary to replace hoses?
For eons the "experts" have warned us you cannot mix the two, glycol and silicone, even a little. I have worked for two OEMs' corporate offices (Honda and Kawasaki) and numbers of authorized dealerships, taken tons of classes (more than I care to remember). And always everyone put the fear of God in us concerning this. And so I did the same with others. You want to know the strange part? You ready? It's crap. Just like all the other insane postering folks assume about this subject, they were wrong. Wrong. It turns out part of the design specification for silicone fluid is that it has to be compatible with glycol. I have even talked with one of the fellows who was on the original Dow development team. And if you dig online you will find this to be true. Silicone was required to be compatible. There are even percentages that are enumerated in the original specification. Well, do I go by that then? No. I don't, though it makes sense. I just can't bring myself to. I still disassemble and wash out the entire system. Call it a bit of caution left over from all the brainwashing I received for decades. Tell you something else. On my own bike I even reused the original seals that had been used previously with glycol. And guess what? They worked fine.
Last edited by mikenixon on Thu Oct 04, 2018 7:13 pm, edited 1 time in total.
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Re: The skinny on silicone brake fluid

#6

Post by mikenixon »

tomk1960 wrote:I've been using DOT 5 silicone brake fluid in all my restorations for years and swear by it. I've heard that if it mixes with DOT3 or DOT4, it will gel and cause problems. In all but one of my restorations, I completely overhauled the calipers, master cylinders, and installed new SS braided brake lines. In the one exception where I overhauled everything but used the original lines, I blew out the lines with BraKleen followed by lots of compressed air. I had no issues whatsoever.

If you decide to switch to DOT5, you will have to disassemble and clean the calipers and masters. You could definitely wash the internals in warm water with a little dish detergent, but why skimp? Just rebuild them with new internals, install new lines, and then you're done for a VERY long time.

There are proponents to DOT5 that will vehemently state that it will destroy seals, etc. Not true and I have experience to prove it. I love it and will never use the other horrible brake fluids again.

And DOT5 isn't as hard to bleed as one might think. Here's proof:https://www.youtube.com/watch?v=m4wlSQDsEHw
Don't call it DoT5. Call it silicone. Read the article.
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Re: The skinny on silicone brake fluid

#7

Post by mikenixon »

One thing I ran across in my research is that silicone fluid may have escaped the control of any governing organization. In fact (and you Euro guys should clarify for us) it appears parts of Europe refuse to acknowledge the existence of silicone fluid. This is why by the way the U.S. and Europe do not agree on the DoT designations. So being unregulated, it's possible there are rogue examples of the stuff out there. What I mean is, different examples of silicone fluid may differ enough to be a problem. I have not confirmed this, but I strongly suspect it based on things I have read. For example: it appears some producers of silicone brake fluid may be adding thinners to it. I am not sure why they do this, but I don't like the sound of that. My feeling is that the thinning solvents may well be the source of some of the bad experiences folks have had. For what it is worth, it appears Tom and I use the very same brand (as it happens), Advance Auto's (online retailer) Carquest brand (made by AGS, an 80+ year old company specializing in brakes).

Back to switching fluid to fluid. I am anal and have gone full clean, as I said. But I am convinced it is not actually necessary. Nor even is flushing. One thing though. Just because you don't need to disassemble or even flush (and I do both) doesn't mean problems in the line present with the old fluid may not crop up and be problems with the new (water, rust, corrosion) and understand it's not the new fluid that is the fault though no doubt it has been blamed. So for my money, unless your system is less than a year old since its last complete rebuild, I would completely disassemble and clean (I run everything in an ultrasonic).
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Re: The skinny on silicone brake fluid

#8

Post by chewy999 »

mikenixon wrote:One thing I ran across in my research is that silicone fluid may have escaped the control of any governing organization. In fact (and you Euro guys should clarify for us) it appears parts of Europe refuse to acknowledge the existence of silicone fluid.
Thanks for another brilliant article Mike. As far as the UK is concerned, (I've just googled it) silicone brake fluid is available but a lot of distributors are labelling it as 'Dot 5'! Can't speak for the rest of Europe.

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Re: The skinny on silicone brake fluid

#9

Post by mikenixon »

chewy999 wrote:
mikenixon wrote:One thing I ran across in my research is that silicone fluid may have escaped the control of any governing organization. In fact (and you Euro guys should clarify for us) it appears parts of Europe refuse to acknowledge the existence of silicone fluid.
Thanks for another brilliant article Mike. As far as the UK is concerned, (I've just googled it) silicone brake fluid is available but a lot of distributors are labelling it as 'Dot 5'! Can't speak for the rest of Europe.

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Thanks for the input chewy.
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Re: The skinny on silicone brake fluid

#10

Post by Snapper33 »

I restored a Morini 3 1/2 and used silicon brake fluid, it’s pretty purple and it stops the bike perfectly. Takes a little longer to bleed correctly, but it’s reliable, long-lasting, non-caustic to my paint and chrome. Love it! I will eventually replace all of my bike’s systems with silicon and be very happy about it.
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