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Old 10-11-2010, 12:22 PM
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synthetic dot 5.1 brake fluid

I'm at the cross road.
I'm installing new master cylinder, calipers and wheel cylinders.
15 year old DOT 3 MC still works fine. All 15 year old wheel cylinders still function fine but I noticed DOT 3 seeping past seals in all wheel cylinders.
I've changed out DOT 3 about once every 5 years.

Plan to buy brake fluid moisture test kit for my DOT 3 cars.

I really wanna take care of new K/H 4 chrome plated piston calipers.

Don't wanna risk DOT 5 silicone because I try to keep my shop silicone free.

I read some posts about DOT 5.1 synthetic brake fluid.

Looking for more DOT 5.1 comments or better DOT 3 maintenance/testing procedure.

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Old 10-11-2010, 01:18 PM
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Hi,
The dot 5 silicon fluid shouldn't be used in anything but a track car.
I don't know a lot about the dot 5.1, but in your case dot 4 would be better then the dot 3 you're using, as the 4 is less hygroscopic & has a higher boiling point wet & dry. Also, 5 years is tooo long between flushing 2 years max.
Rich
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Old 10-11-2010, 01:44 PM
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Quote:
Originally Posted by richard stewart 3rd
Hi,
The dot 5 silicon fluid shouldn't be used in anything but a track car.
I don't know a lot about the dot 5.1, but in your case dot 4 would be better then the dot 3 you're using, as the 4 is less hygroscopic & has a higher boiling point wet & dry. Also, 5 years is tooo long between flushing 2 years max.
Rich
"5 years tooo long"...I was afraid you were gonna say that.

I didn't know DOT 4 was less hygroscopic.
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Old 10-11-2010, 03:22 PM
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Silicon was first used in motor sports with hill climbers. The choice was due to the fact that silicon has a higher boiling point. They soon learn that silicon is non-hygroscopic which is not good in a brake system.

Problem was moisture exist in the air and suspension. It is next to impossible to completely isolate air. Even opening a can of fluid introduces some moisture.

With a hygroscopic fluid (non silicon) fluid is absorbed and distributed through out the system. A brake system can absorb and disperse 1-2 teas spoons of moisture without brake failure. With non-hygroscopic fluid, as much as one drop will fall out of suspension to the lowest spot (the calipers) and cause brake failure.

When the hill climbers got the brakes hot, they soon lost their brakes. It did not deter them or others in motor sports from using it BUT when they do, they change their fluid before every event. Non silicon is suppose to be changed every 1-2 years (when did you last change yours?).

People with parade cars, and really low performance cars, VW's, MG's etc love silicon because it does not attack paint; but at the same time they never get the calipers hot enough to matter.
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Old 10-13-2010, 10:05 AM
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prestone synthetic DOT 4

Prestone synthetic DOT 4 has all the correct buzz words.

http://www.prestone.com/products/print/440?popup=1

I am more interested in corrosion inhibitors than dry boiling point.

Less hygroscopic would be nice also.
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Old 10-13-2010, 10:08 AM
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Hygroscopic is more of a catch phrase. Silicon is non Hygroscopic so water settles out of suspension. Hygroscopic brake fluid spreads out the moisture so it's not a threat in the brake system.
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Old 10-13-2010, 10:30 AM
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Think of it this way. Take a glass jar of Lacquer thinner (non hygroscopic) and set it in your shop without a lid. If there is any humidity, with in a day or two it will still soak up water from the air it but the water settles in the bottom of the glass jar. Within a day or two you see water bubbles in the bottom. This is what would settle in your calipers.
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Old 10-13-2010, 11:50 AM
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Copy that.

Perhaps some brake fluids are hygroscopic but to a lesser extent that typical DOT 3.

I don't water to fall out of suspension. I would like water to be absorbed at a slower rate.

Bottom line: I prefer brake fluid to be less corrosive if possible.

I suppose it would help to minimize air space in reservoir. Fill to at least max fill line.
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Old 10-13-2010, 11:59 AM
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They say rubber lines can absorb some moister as well. I don't know that if that is true or not. Industry seems to equate Hygroscopic to:

1. The ability to pull from the atmosphere
2. The ability to dissolve and distribute in the fluid

I wish there were some studies which would measure this a little better.

It has been long standing that traditional brake fluid should be changed 1-2 years depending on who you ask.

I know mine certainly goes much longer then that, as do most. When some one goes 25 years without changing and has brake problems I don't think it a reflection on the fluid.
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Old 10-13-2010, 01:46 PM
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my data

66 gmc probably did go 25 years before MC failed. wheel cylinders rarely changed when leaking. fluid never changed except when WC replaced. Yes fluid looked like swamp water.

my mustangs get a fluid change roughly every 5 years. Both mc ok after 15 years. all wheel cylinders started leaking between 10 to 15 years. Fluid looked pretty good; little murky at WC. Some deposits found in MC and WC.

1990 fzr 600 fluid changed once. Everything still working. Fluid started looking rusty.

I'll put it on my list to change fluid every 2 years...uugh.
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Old 10-13-2010, 02:19 PM
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Quote:
Originally Posted by 001mustang
I'll put it on my list to change fluid every 2 years...uugh.
LOL, yea it makes the list on mine as well. It keeps getting shifted down the list though. Last time I had my truck in, I had the shop do it. I hate doing it. I can't do it if I am in the paint process on a car.

What engine did you have on the 66 GMC?
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Old 10-13-2010, 04:26 PM
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1966 GMC 305E V6.
Big Block V6.
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Old 10-13-2010, 07:34 PM
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Brake Fluid, All you need to know

Do not use a silicon brake fluid (DOT 5) in your cars, even
after a complete flush. You can use DOT 5.1 however.
Here's why.

Brake fluids are classified by both “dry boiling point” and
“wet boiling point” among other things. DOT 3 has minimum
dry/wet boiling points of (degrees Fahrenheit) 401/284, DOT
4 is 446/311, and DOT 5 is 509/356. DOT 4+ was used by
manufacturers to designate an ester based DOT 4 fluid that
met DOT 5 specs. But that was confusing so DOT 5.1 is now
used instead of 4+. Dot 4+ and DOT 5.1 are the same thing. It is also called "Super Dot4".

DOT 3, 4, 4+, and 5.1 brake fluids are ether based and are
hygroscopic. ie. they adsorb water. Since water boils at 212
degrees Fahrenheit the adsorbed water dramatically lowers
the boiling point of the brake fluid. A small amount of
water suspended in the fluid decreases the boiling point as
much as 1/3. Plus, water in the fluid is what causes
corrosion in the brake system.

DOT 5 Silicone based brake fluids are non hygroscopic and
that takes care of the reduced boiling point and corrosion
problems associated with moisture being absorbed by the
ether based DOT 3, 4, 4+, and 5.1 brake fluids. That's why
they are specified by the U.S. Military. Unfortunately the
silicone based fluids are compressible themselves so they
produce a soft pedal all by themselves. In addition,
Silicone based fluids allow air to become dispersed in the
fluid leading to even spongier feel tot he brakes. For the
person who doesn’t care about a spongy pedal or precise
braking silicone fluids may okay but not for anyone reading
this.

Generally speaking, you can use a higher DOT number (EXCEPT
DOT 5) in any application, but you should not use a lower
number. ie. DOT 4, 4+, or 5.1 in a DOT 3 application is
fine, DOT 4+ or 5.1 in a DOT 4 application is fine.
However, you will not get any better performance from
upgrading your fluid. There will be no performance gain
unless you are racing the car on a high speed road course
and the fluid is overheating.

All that said, I suggest you use ATE Super Blue Racing
and ATE TYP 200 fluids. They are identical except for
color. TYP 200 is yellow. I alternate fluids when I flush
and replace fluid because you can see by the color when the
old fluid is flushed.

For a real education on brakes, go to

http://www.stoptech.com/

and read their white papers. It is the best source I know.
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Old 10-13-2010, 07:42 PM
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Quote:
Originally Posted by WildBill
DOT 5 Silicone based brake fluids are non hygroscopic and
that takes care of the reduced boiling point and corrosion
problems associated with moisture being absorbed by the
ether based DOT 3, 4, 4+, and 5.1 brake fluids.
Since the moisture in suspension is not absorbed it falls to the lowest point in the braking system; the calipers. While non-silicon fluid can absorb 1-2 teaspoon in a braking system before it is a problem, one drop of water, settling into the caliper causes brake failure pretty much at the 212 degrees. That is why the racers who used it soon found that it had to be changed before every race.
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