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Old 04-09-2002, 03:30 PM
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Post Dot 5 Silicone Brake Fluid?? Yes or No ??

I recently went to a web site that designed and sold brake parts exclusivly. I read the following information about dot 5 silicone brake fluid on their web site.

Silicone based fluids are great for museum cars that are never driven, because the fluid is very non-reactive. The moisture that is bound to eventually get into the brake lines will not dissolve in silicone fluid. This water forms pockets that will boil off and become steam as the brake fluid heats up, and as a result it is virtually impossible to keep a silicone based system bled properly as there will always be gas in the lines.

I have been running dot 5 silicone based brake fluid in one of my cars for 20 years, drive the car almost every day, and I have never once experienced any of the problems described above, or any other problems for that matter, with this brake fluid!

Has anyone else had any experience with dot 5 silicone based brake fluid, good or bad????????????

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Old 04-09-2002, 05:44 PM
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I'm no expert on DOT5 fluid but I'm pretty sure some European sports cars come with DOT5 from the factory. Might be worth looking into, I've always used DOT4 because you can use it to flush the DOT3 out of the system. 3 and 4 are compatible, just not 5.
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Old 04-09-2002, 06:22 PM
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When I rebuilt my '40 Buick in winter of '00, I installed all new brakes front and rear, new lines, and new Master Power Brake booster, master clyinder, and prop. valve. It was my intentions to change over to Dot 5 fluid at this point since all was new, until I read the warranty on the MPB parts. It was VOID if I used silcone fluid. Naturally, I regressed to the old standby, Dot 4 and have lived happily ever after.I have never really had a problem with it,or Dot 3, except one time some 25 or so years ago on a brand new 1976 Harley Electroglide. The Dot 3 fluid would "boil away" from the rear calipers and you would have NO brake...fixed it by going to DOT 5 fluid, after a 2500 mi trip to Florida and back with no rear brake. I was younger and braver back then(read stupid).
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Old 04-09-2002, 06:29 PM
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Of critical importance in determining a fluids ability to handle high temperature applications is the Dry Boiling Point and compressibility.


The Dry Boiling Point is the temperature at which a brake fluid will boil in its virgin non-contaminated state. The highest temperature Dry Boiling Point available in a DOT 3 fluid is 572 F.

The Wet Boiling Point is the temperature a brake fluid will boil after it has been fully saturated with moisture. The DOT 3 requirement for wet boiling point is a minimum temperature of 284 F.

There are many ways for moisture to enter your brake system. Condensation from regular use, washing the vehicle and humidity are the most common, with little hope of prevention. Glycol based DOT 3 & 4 fluids are hygroscopic; they absorb brake system moisture, and over time the boiling point is gradually reduced. Here is a scary statistic, a new car at 12 months old will have a 2% water content if never driven, at 18 months it will be 3% or higher depending on atmospheric conditions. It is not unusual to have 8-12% water content in the average automobile on the street.

You should not use DOT 5 fluid in any racing application for the following reasons. DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers. It is not uncommon to have caliper temperatures exceed 200 F, and at 212 F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel. DOT 5 fluid is best suited for show car applications where its anti-corrosion and paint friendly characteristics are important.

Whenever you add fresh fluid to your existing system (never mix fluids of different DOT classifications), it immediately becomes contaminated, lowering the boiling point of the new fluid. For maximum performance, start with the highest Dry Boiling Point available, flush the system completely, and flush it regularly, especially after severe temperatures have been experienced.

If you happen to mix DOT 3/4 and DOT 5 you will end up with a coagulated gel that will seriously compromise your braking performance. Dot 5.1 is a Glycol based fluid like DOT 3 or 4 and should not be confused with DOT 5 which is silicone based, many new European cars specify DOT 5 but in Europe they don't recognize the silicone based (DOT 5) fluid so they really mean DOT 5.1, ensure you don't confuse the two. DOT 5.1 was developed for ABS braking systems and is of a thinner viscosity to allow shorter cycle timing, it's boiling point is similar to DOT 3 or 4.

Much of this info I cut and pasted from the Wilwood brakes site and I have had similar expierience with GT3 racing cars when I tried DOT 5, it was a waste of time and money.

[ April 09, 2002: Message edited by: 4 Jaw Chuck ]</p>
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Old 04-09-2002, 07:49 PM
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This is for 4 Jaw Chuck, just wondering about you saying (never mix fluids of different DOT classifications). I know you can't put DOT3 into a DOT4 system because DOT4 is rated higher. It would be like putting type F trans fluid in a new car calling for MerconV. But I have a bottle of Castrol GTLMA brake fluid in front of me. It says right on it 'exceeds DOT3 and DOT4 specifications and mixes with both'. It's more expensive so we don't generally use it unless the system calls for DOT4 but sometimes if we're out of DOT3 at our station we'll use the DOT4 until we get more from parts dept. It says right on the bottle mixes with all conventional brake fluids (DOT3 & 4). Yes if you add DOT4 to DOT3 you are lowering the boiling point of the DOT4, but if the system only requires DOT3 you're still ahead of the game.
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Old 04-09-2002, 09:42 PM
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Thats not what I meant, DOT 3 or 4 mix together just fine and are interchangeable with DOT 5.1, these are all glycol based fluids with different temperature ratings and viscosities.

You can't mix DOT 3,4,5.1 (glycol) with DOT 5 (silicone) you will get a jelly, give it a try and see for yourself.

[ April 09, 2002: Message edited by: 4 Jaw Chuck ]</p>
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Old 04-10-2002, 03:06 AM
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You know I kind of figured that, just how it was writen. I'm just a sucker for detail sometimes. I just have to assume that there are some out there reading this that have never even seen a brake pad and are just now learning. It can get confusing with the different types and like I said I've mixed DOT4 in many DOT3 systems. Oh well off to work I go!
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Old 04-10-2002, 06:51 AM
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Dot 5 should in my opinion not be used on anything(not even your john deer lawn tractor).It does not work in a race car.Just ask the guy that was in front of me all nite on the local short track(he did not have much rear bumper left )I have fixed many brake problems on rods also by simply removing the dot 5 . I have heard a few success stories using Dot 5 mostly from some of the SCCA vette boys but most of them payed to have there work done and i suspect they were getting 4 and paying for 5.. I have allways used the Castrol GTX dot 4 product and have no problems.(nope i dont work for them) If you mix dot 3 with 4 it lowers the boiling point.The boiling point on Dot 4 comes down to what Dot 3 is in a matter of months so if you dont do the bleed and flush deal every year there is probably no great gains in using Dot 4. The pedal pressure alone would stop me from every using Dot 5 again.If you ever had to drive down into a corner in the triple digits and pushed on the middle pedal with dot 5 you would never use it again. LOL Its great selling point is it does not eat paint. Well a little tip ...dont put it on the paint Now this is a real tip ...not kidding.Buy brake fluid in small containers(no swap meet crap,or gallon jugs)After you break the seal and use it toss the rest out.If its on the shelf for a few months its not worth putting in your car. I could go on and on about Dot 5 as i was sponsered by a company that makes the stuff ,but we could just not get the stuff to work.Brakes would hang up on a 50 lap race.You would have to stand on the brake pedal to get it slightly slowed down.We finally kept the stuff in the hauler but would make sure the brakes were bleed at the shop(were no one saw we were not using the product) I am no engineer but it wont get used in this shop again.

[ April 10, 2002: Message edited by: Phat ]</p>
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Old 04-10-2002, 12:34 PM
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somebody needs to explain why dot 5 requires more brake pedal force than dot 3 or 4. One would think that all hydraulic fluids would compress the same therefore reqiuring the same pedal force.
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Old 04-10-2002, 02:03 PM
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When I used DOT 5 it did not cause higher pedal pressure only a mushy feel because it is slightly more compressible than DOT 3 or 4. With the mechanical advantage at the pedal it becomes very noticeable.

Higher pedal pressure could be caused by the coagulation of DOT 3/4 and DOT 5 if the systems were no properly diassembled and cleaned with alcohol to remove the last traces of the incompatible fluid.

Despite what they teach you in school liquids are compressible, but the amount is miniscule compared to gases.
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Old 04-10-2002, 02:33 PM
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I really did not mean to say pressure ...i guess i meant to say travel.If you have driven with dot 5 you will know what i mean.Your leg will get really tired quick with all the pumping your doing.You are correct it is like stepping on marshmellows. Sorry but i dont splain stuff so good.
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Old 04-13-2002, 04:01 AM
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On my home built Model A I used DOT 5 silicone brake fluid from the beginning. I have not had any problems with the braking system in the five years it has been on the road.

I am using a Mustang non-power master cylinder, GM front disc calipers with a Midland booster, GM drum rear, and have an adjustable proportioning valve in the rear line.

I will admit, it doesn't have the "throw you through the front windshield" feel that you get in a new car. Mostly because the Midland booster isn't doing much, running a prety high lift cam, there isn't much vacuum available.
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Old 04-13-2002, 09:02 PM
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In the early 80's the US Postal Service was one of the first vehicle fleets to ban use of asbestos brake linings. With the replacement brake parts manufactures, knowing that in short few years the manufacture of asbestos brake was to be ban by law,the Postal became one of the "test beds" for the soon to be required "non-abestos" linings.About the sametime it also took on the use of "DOT 5" brake fluid.Unfortuantely the brake failures & maintenance increased about 75-80%.Every thing from leaks, to loss of pedal. A carrier would call in Saying he had "no brake pedal". By the time the vehicle was towed in and the next day or so we got to check it out, the brakes would be fine.Closer investigation reveal Silicone fluid the culprit, not the originally accused "Non- asbestos" linigs.
It's a "DOUBLE DAMN"--in the case of a Postal carrier or as PHAT said a race car driver's repeated use of the brake was double trouble.Silicone fluid does not absorb water but it does absorb "air"and the brake system being vented the more the pedal was used the more the fluid became "aerated" so the pedal became "spongy" or even went away completely. After the vehicle set for awhile and all the air bubbles disappeared the brakes were fine. The "Double Damn" was that when the air got into the fliud,the tiny bubbles created were very "abrasive"which lead to failure of master & wheel cylinder rubber parts & their bores(especially aluminum M.C.'s).Silicone brake fliud is also affected by altitude and as stated by many brake part manufactures & Chysler Corp. for one: "Warranty Void if DOT 5 fliud is used". "What's" a little"bubbled" paint when DOT 3&4 fliuds are in less question and probably "Safer". Paulweldit

[ April 13, 2002: Message edited by: PAULWELDIT ]</p>
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Old 04-14-2002, 04:51 AM
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Now this aint a trick question.. i really dont know the answer.Why do all the new bikes use it??? I really dont know.!! Have they figured out something or????
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Old 04-14-2002, 06:50 AM
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That's an interesting qustion by Phat, I don't really know but I'll take some guesses. The first thing it says in my Harley Davidson sevice manual is never use anything but denatured alcohol to clean system components, any normal mineral based cleaners are a definate no-no. Which I think means if you're switching over a DOT4 system to a DOT5 cleaning and replacing almost everything is critical. Any old chemial or cleaning agents in that system will cause trouble. That might explain why a new system built clean that never had anything but DOT5 in it would be less trouble. My bike is 11 years old with DOT5 fluid in it, so I know Harleys been using DOT5 for at least that long. One other observation I made is the reservoir covers on my bike are both screwed and sealed tight to the reservoirs. This might allow less air to penetrate the system. I think PAULWELDIT mentioned DOT5 absorbs air. The last thing you want on a 100hp two wheeler is bad brakes and all they use is DOT5. Go figure? <img src="confused.gif" border="0">
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