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Silicone Brake Fluid


John N. Packard
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To get the best results, put all new quality brake parts in (rubbers) and flush all lines to clean out the old fluid. Keep a bottle in the trunk on trips in case you need to add some. <BR>I have used it in my '33 Street Rod for over 20 years and when you hit the brakes the first time in the spring...you have solid brakes. We also have had it in a '72 <BR>Vette for 15 years and never have any problems with it. I highly recommend it for cars that sit a lot, as it doesn't draw moisture.

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When I put my 1933 back together I used silicone brake fluid. As I understand it there are two draw backs to silicone fluid:<BR><ol>[*]It is easy to get air bubbles in it while you are bleeding the brakes. Going slow and easy works around that.<BR>[*]At very high temperatures is produces a spongy pedal. For that reason racing drivers don't like it. For "normal" everyday driving, much less careful antique car touring, the brake temperatures don't get that high. We aren't talking about repeated full power acceleration followed immediately by hard braking for hours on end as you get in racing. At least not how I tour smile.gif<BR>

The advantages for a collector car are considerable:<ol>[*]It does not absorb moisture from the atmosphere. During normal temperature cycling, even day to night while parked, the brake system breaths. Conventional brake fluid absorbs water during that cycle.[*]It does not damage paint.[*]I have heard, but don't know for sure, that a coating of silicone brake fluid will actually protect metal from rust.

For a car that is parked most of the time and expected to remain functional for many years, I think silicone brake fluid is the way to go.<P>Just my $0.02 worth.<P>Tod

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It's funny that you mention the stop light switch problem. When I redid my '64 1/2 Mustang, I used silicone brake fluid because of all of the reasons above, but I, too, go through switches about once every 18 months. I even cut one apart to see what failed, but found it to be normal. The nearest I can tell, is that the fluid was allowed to get behind the diaphram. This prevents the diaphram from closing the switch contacts. The diaphram is supposed to be impervious to silicone fluid, but they still fail. I can count on a failure so regularly, that I replace it every spring just to keep it from failing on the road.

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I have used silicone brake fluid for many years in both antique and modern cars with only the brake switch problem in one car.However, there was a recent article in Skinned Knuckles magazine stating that in cars with a vacuum power brake unit if there is a leak in the diaphram and the fluid is drawn into the engine and burned it turns into an abrasive.

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Thankfully, I don't have a power booster to worry about on the 1933. It does, however, have a hydraulic light switch. The above comments about failures in the switches has me concerned: What is the failure mode? Does the lamp cease to turn on? Or does the hydraulic system fail any you end up with no brakes?<P>If the failure is simply a loss of lamp action, I can deal with it. If the pedal ends up going to the floor when the switch fails, I will change the setup.<P>By the way, I have about four years on the switch, with no sign of problems. The system has seen nothing by silicone fluid since is was rebuilt from scratch (new cylinders, tubing, etc.).

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Hello you all.<BR>I read this discussion about the silicone brake fluid and I remember that we had some warnings here in Germany about it. At first, the silicone fluid is of course the better fluid as the normal with all the pros you wrote down. Here they say the same but they say also that it is important that all brake parts (hoses, rubbers in the cylinders) must be developed for the silicone fluid. Otherwise the rubbers come in a bad condition.<BR>That means (as far as I know) you have to flush the brake system and to change the rubber parts of the brake system.<P>Thomas

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In my Mustang, the only failure was the lights failed to come on, but then, I do not have power brakes. The question is, "Why does this only happen to the switch and not the other rubber parts in the system, like wheel cylinder rubbers." The new parts today are not affected by silicone fluid. I sent my disassembled switch to Bill Canon of Skinned Knuckles when it failed, but I never heard back from him. As near as I can tell, the fluid got behind the diaphram and there was no longer a pressure differential pressent to cause the switch to operate. This would indicate to me that the diaphram shrunk just enough to allow the fluid to get behind it. The fix is easy enough, but I don't understand why it happens.

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Silicone brake fluid will creep thru almost any seal.When it gets into your brake switch , and gets into the contacts it becomes a dieletlic{insulator}the switch ceases to conduct the electric current for the stop lites.

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I have had silicone fluid in my 51 Pontiac since I bought it in 1998. The best thing, especially for cars that sit, is that the rubber does not swell up in the brake system. I have been trying to figure out why my hydraulic switches keep burning out. I guess you guys answered my questions. I thought it was just that th enew ones from napa aren't made as good as the original. I have been to at least 4 or so since I have had this car. A friend of mine even went so far as to just put on a mechenical switch on his car ( he has a rod though). A friend of mine was sick and didnt drive his 49 chevy for 3 months and when he did, he had brake problems. He wasn't using silicone fluid. Al the rubber was shot. So he went though the system and replaced the fluid with silicon. No problems yet. 49 Chevy has a mechanical switch too, so he doesnt have to worry about that I guess.

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Someone earlier said that the switches they were using were suppose to be rated for silicone fluid but still didn't last. Maybe you could find one at a motorcycle shop. Most motorcycles use silicone fluid. I don't know why they would be made any different from automobile swiches, but the one on my motorcycle has been on there for 15 years in silicone fluid with no problems.

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  • 3 weeks later...

Back to the top on this one. <P>Yesterday I got to thinking about the hydraulic stop light switch in my '41 Cadillac since I use silicone brake fluid in that car. I wondered if it still worked.<P>Up to the minute report for what ever it is worth to anyone.<P>Switched to silicone brake fluid in 1986 after doing all of the requisite chores like new cylinders [wheel and master], flushing all lines and installing new stop light switch.<P>Checked stop lights at 2:00PM, March 28, 2001.<P>The stop lights work perfectly after the switch having been immeresed in silicone for 15 years. cool.gif<P>hvs smile.gifsmile.gif<BR>

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Good response from Mr. "Ask the Man Who Owns One." John, another 15 years on that switch would be good, but I'm afraid it was not a genuine Cadillac replacement part. Just something I picked up at Federal Auto Parts in south Baltimore. smile.gifsmile.gif Howard

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Had the same problem with the stop switch in my '46 gmc truck. Found that some trucks of same era used a mechanical switch that is mounted on the engine side of the firewall and is actuated by the brake arm. A small price to pay to derive all the benefits of silicon.

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  • 9 years later...

Looks to me like this is a silicone fluid feedstock that would be used by other manufacturers as a base to blend with other componants to make an end use specific product such as brake fluid.

You can't go wrong by just going to your local NAPA store and buying what you know is brake fluid.

BTW, in my opinion silicone fluid is far superior to DOT 3, especially for cars not regularly driven................Bob

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  • 2 weeks later...

I have used it in all my old cars as well as surge brakes on trailers. When I clean the system and rebuild everything, I automatically install a GM style switch on the brake pedal. It is usually not too hard to engineer a bracket. I have found that the pressure switches do not last long---even the ones that claim to be OK for silicone fluid.

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I'm one more fan of silicone fluid. Had it in my 30 Desoto system for almost 20 years with no troubles except stoplight switches which are easy enough to change.

when a switch goes it does not affect braking action.

Be sure to use all new rubbers including the 3 flex hoses feeding the front and rear wheels and do flush the old hydraulic lines. Old hydraulic lines may have rusty insides so new steel lines are a better idea.

Martin Lum

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  • 1 month later...

Silicone break fluid DOES NOT absorb more water !

That's one of the big reaons to use it in the first place !

I put Silicone Dot 5 in my MGTD in 1972 !

I was one of the very first to use it in my area. It was very hard to find in those days.

Wheel cylinders in the MG are aluminum lockheed units.

The system is as good as the day I installed it !

Corvette and many high-end sports cars use silicone !

It also has a much higher boiling point !

I use nothing else in my old cars~

Edited by Silverghost (see edit history)
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What about this? We all buy a "new" antique car occasionally. In many cases the previous owner of the car no longer remembers, is deceased, or for some other reason just does not know what type of brake fluid is in the car.

Is there any way to tell which type of fluid is in the braking system? I have 3 cars at present in which the type of brake fluid in the car is unknown and eventually will have to add brake fluid, so am wondering what to do?

Also lets assume that a car has Silicone brake fluid and (since we don't know), do a brake job and add Dot 3 brake fluid during the bleeding process. What happens then? (Or car has Dot 3 and we add Silicon brake fluid). Certainly this happens on occasion and when it does, what are the results?

Thanks, Fred

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What about this? We all buy a "new" antique car occasionally. In many cases the previous owner of the car no longer remembers, is deceased, or for some other reason just does not know what type of brake fluid is in the car.

Is there any way to tell which type of fluid is in the braking system? I have 3 cars at present in which the type of brake fluid in the car is unknown and eventually will have to add brake fluid, so am wondering what to do?

Also lets assume that a car has Silicone brake fluid and (since we don't know), do a brake job and add Dot 3 brake fluid during the bleeding process. What happens then? (Or car has Dot 3 and we add Silicon brake fluid). Certainly this happens on occasion and when it does, what are the results?

Thanks, Fred

The horror stories I've heard about DOT5 (silicone) versus other types of brake fluid have been when they are mixed, so I'd really try to figure out what is in that "new" old car.

Out of the container the DOT5 fluids I have are purple. I don't know if that changes over time in the brake system or not. But I think a sure fire way to tell what is in the system would be to take a little fluid from the master cylinder and put it on a painted surface: DOT5 will not damage paint while DOT3 will.

In addition to being good for use in vehicles that are stored for long periods of time, the fact that DOT5 won't damage paint was one of my considerations when I used it in my old car.

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silicone brake fluid absorbs more water.

"rjohnson67",

You site a home page from a very complicated, technical site, who's base page is addressing "Silicone Rubber". How is this pertinent to the discussion of "Silicone Brake Fluid"?

This page has the statement - "Silicone rubbers are inorganic synthetic elastomers made from a crosslinked silicon-based polymer reinforced with filler." There are no elastomers in "Silicone Brake Fluid" and no reinforcement with filler either! I suggest the "filler" may have something to do with your comment, but more specifics from you are warranted than just the comment "silicone brake fluid absorbs more water".

Your comment is contrary to over 20 years of research and experience by those in the car and motorcycle hobbies concerning "Silicone Brake Fluid" and just siting the home page of a technical site does not support your comment much.

More specifics are needed or your comment would have to be considered invalid.

Here is some more info linked from the same site:

"Why silicone materials excel in automotive applications...

Silicones have unique properties that enable them to thrive in harsh engine compartment, under-vehicle, weather, and driving conditions:

They are stable over a wide range of temperatures, from -50 to 200°C (-58 to 392°F) and resistant to thermal shock. This enables flexible joint movement with excellent sealability.

Silicones resist moisture, pressure, salt spray, engine fluids, and UV light.

They have stable dielectric properties, even at high frequencies.

Silicones have very low water absorption: 0.1% per weight at room temperature.

Silicones have excellent sound and vibration damping capabilities."

http://www.dowcorning.com/content/discover/discovershowcase/automotive.aspx?bhcp=1

Edited by 1936 D2
Added more specific info. (see edit history)
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I really cannot understand why the entire auto industry has not as yet switched to silicone brake fluid ?

My guess is their dealer repair shops make too much money doing Dot 3-4 hydraulic repairs.

AND~

The parts suppliers also sell more master cylinders & wheel cylinders & calipers this way using old style fluid !

I once was talking to an old time car mechanic who stated that he thinks silicone brake fluid makes the brake pedal feel spongy !

"Tried it~ Didn't like it !"

I replied that that was impossible !

ANY fluid will not compress !

Water, Dot 3-4 conventioal, oil, trans fluid etc.

They should all feel the same if the system was properly bled of all air ...

He still did not believe me !

Silicone brake fluid is still misuderstood some 30+ years later !

I cannot understand this !

I would use nothng BUT Silicone from now on !

Edited by Silverghost (see edit history)
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I really cannot understand why the entire auto industry has not as yet switched to silicone brake fluid ?

I once was talking to an old time car mechanic who stated that he thinks silicone brake fluid makes the brake pedal feel spongy !

"Tried it~ Didn't like it !"

As far as I know, silicone brake fluid is not compatible with ABS equiped vehicles. I cannot prove it, it just seems that the lubrication properties of the silicone fluid do not match those of the DOT 3 or 4 brake fluid.

Silicone brake fluid can be very tricky regarding air. You have to be very careful how you pour the fluid into the master cylinder: it can trap millions of tiny air bubbles giving the brake pedal that spongy feel.

Another bad habit of the silicone brake fluid: the flared ends of the lines must be very well formed to avoid leaks; if one or more flares is not 100%, a conventional brake fluid would not leak but silicone fluid would.

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I use just the usual DOT 3 and flush the system every 5 years or so. But, water absorbtion is not a threat in Arizona, as in some locations. I read some time ago that silicone brake fluid is hazardous in mountain areas, because it has a propensity to collect air bubbles with elevation changes. Has anyone heard this as well?

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I use just the usual DOT 3 and flush the system every 5 years or so. But, water absorbtion is not a threat in Arizona, as in some locations. I read some time ago that silicone brake fluid is hazardous in mountain areas, because it has a propensity to collect air bubbles with elevation changes. Has anyone heard this as well?

Air bubbles get in when the fluid is agitated so you have to be careful when filling the master cylinder. I don't see a mechanism where the fluid would entrain air bubble by cycling the barometric pressure. My guess it that it is a "urban legend".

For what it is worth, I live very close to sea level and have driven my silicone brake fluid equipped 33 into the mountains at elevations up to 9,000 feet on several occasions with no issues due to brake fluid. Mind you I am careful to use engine "compression" braking on down grades to avoid brake fade with those tiny 10x1.5 inch drums. But that is not an issue with the type of fluid, all internally expanding drum brakes will have fade issues if the drums get too hot.

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Silicone brake fluid can be very tricky regarding air. You have to be very careful how you pour the fluid into the master cylinder: it can trap millions of tiny air bubbles giving the brake pedal that spongy feel.

True! And since we all know that, we use extra precautions when filling and bleeding the brakes. We also are a bit more cautious with cleanliness as we work with the open system brake parts. Both of these are good ideas for anyone working on these critical systems.

Another bad habit of the silicone brake fluid: the flared ends of the lines must be very well formed to avoid leaks; if one or more flares is not 100%, a conventional brake fluid would not leak but silicone fluid would.

I did the rebuilding of my brake system back in 1986 as I recall. I had NO experience doing this. I was learning along the way.

I disassembled the wheel cylinders and master cylinder, cleaned all those parts (twice), honed both the master cylinder and all eight parts of the stepped wheel cylinders (learning about brake cleaning fluid and how the hone worked along the way). I found some (very expensive) DOT-5 fluid (not easy to do at the time) through a motorcycle parts supplier.

I taught myself how to use my brand new "mini pipe cutter" and "double flare tool" on the new steel brake line stock and made all new lines except for the one across the front that was buried in the frame members. (That one was cleaned three times with ethanol and then rinsed with DOT-5). I purchased three new brake hoses (not necessarily rated for DOT-5 because it was so new at the time) and used off-the-shelf new wheel and master cylinder kits (also not necessarily rated for DOT-5) to finish the job. I am proud to announce that I had NO LEAKS the first time! I was looking critically for leaks because of what I heard about silicone fluid finding the smallest places to leak from. I only recently (within the last year) had to replace any fluid in the vented master cylinder. It was about 1/4 down. I had looked each year at the fluid level in the master and found it NEVER dropped so after about 5 years I got a bit complacent about checking the brake fluid level. So, somewhere in the next nine years the system dropped the 1/4 requiring the fill.

Now I would say that is all pretty good considering all the horror stories out there about how bad Silicone Fluid is supposed to be. BALDERDASH!

I feel this is proof positive that DOT-5 fluid is THE BEST to use in the old car application because of the special conditions of a lot of "non-use" of the vehicle and through A LOT of wide temperature changes (from highs of around +98 degrees to lows of -25 degrees) during this long term storage and also wide variations in relative humidity (from highs of 100% to lows around 15%).

This all shows you that it does not take some special high level of mechanical knowhow to do a complete successful brake rebuild using DOT-5 fluid! It just take some PATIENCE and careful thought as you go along. Understand what you need to do and then apply that understanding carefully. It can be done! ;)

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