Re: Battle of the DOTs

[chymar01@xxxxxxxxxxx]

While I understand what the article is saying about attracting the crud in the system when DOT 5 is introduced(section #4), the fact remains that the fluids themselves are fully miscible. Federal law requires that all brake fluids sold in the US must be able to work with each other. That way, if someone needs fluid NOW, they can use whatever is on the shelf. 
It is recommended that when switching to DOT 5 that it be done when replacing the brake system components all at once. I wouldn't switch otherwise. And definitely crack the bleeders at least once a year like it says below. 


----- Original Message ----- 
From: "Herb" <zephyr9900@xxxxxxxxxxx> 
To: "1962 to 1965 Mopars" <1962to1965mopars@xxxxxxxxxx> 
Sent: Wednesday, March 11, 2009 5:54:07 PM GMT -05:00 US/Canada Eastern 
Subject: Battle of the DOTs 



FYI if any one wants to read it! Her it is. 

Battle of the DOTs 
DOT 3-4 Verses DOT 5. Which brake fluid should I use? 
"With regards to the DOT 3-4 verses DOT 5 brake fluid controversy, here is 
an article sent to me by Mr. Steve Wall. It is one of the most professional 
treatments I have seen on the subject". 
[I had to condense this article from 6 pages to 1 due to space limitations. 
Brake Fluid Facts 
by Steve Wall 
As a former materials engineering supervisor at a major automotive brake 
system supplier, I feel both qualified and obligated to inject some material 
science facts into the murky debate about DOT 5 verses DOT 3-4 brake fluids. 
The important technical issues governing the use of a particular 
specification brake fluid are as follows: 
1. Fluid compatibility with the brake system rubber, plastic and metal 
components. 
2. Water absorption and corrosion. 
3. Fluid boiling point and other physical characteristics. 
4. Brake system contamination and sludging. 
Additionally, some technical comments will be made about the new brake fluid 
formulations appearing on the scene. 
First of all, it's important to understand the chemical nature of brake 
fluid. DOT 3 brake fluids are mixtures of glycols and glycol ethers. DOT 4 
contains borate esters in addition to what is contained in DOT 3. These 
brake fluids are somewhat similar to automotive anti-freeze (ethylene 
glycol) and are not, as Dr. Curve implies, a petroleum fluid. DOT 5 is 
silicone chemistry. 
Fluid Compatibility 
Brake system materials must be compatible with the system fluid. 
Compatibility is determined by chemistry, and no amount of advertising, 
wishful thinking or rationalizing can change the science of chemical 
compatibility. Both DOT 3-4 and DOT 5 fluids are compatible with most brake 
system materials except in the case some silicone rubber external components 
such as caliper piston boots, which are attacked by silicon fluids and 
greases. 
Water absorption and corrosion 
The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates 
is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol 
antifreezes, are readily miscible with water. Long term brake system water 
content tends to reach a maximum of about 3%, which is readily handled by 
the corrosion inhibitors in the brake fluid formulation. Since the 
inhibitors are gradually depleted as they do their job, glycol brake fluid, 
just like anti-freeze, needs to be changed periodically. Follow BMW's 
recommendations. DOT 5 fluids, not being water miscible, must rely on the 
silicone (with some corrosion inhibitors) as a barrier film to control 
corrosion. Water is not absorbed by silicone as in the case of DOT 3-4 
fluids, and will remain as a separate globule sinking to the lowest point in 
the brake system, since it is more dense. 
Fluid boiling point 
DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3 (401F) 
and both fluids will exhibit a reduced boiling point as water content 
increases. DOT 5 in its pure state offers a higher boiling point (500F) 
however if water got into the system, and a big globule found its way into a 
caliper, the water would start to boil at 212F causing a vapor lock 
condition [possible brake failure -Ed.]. By contrast, DOT 3 fluid with 3% 
water content would still exhibit a boiling point of 300F. Silicone fluids 
also exhibit a 3 times greater propensity to dissolve air and other gasses 
which can lead to a "spongy pedal" and reduced braking at high altitudes. 
DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of 
such a mix being a lowered boiling point. In an emergency, it'll do. 
Silicone fluid will not mix, but will float on top. From a lubricity 
standpoint, neither fluids are outstanding, though silicones will exhibit a 
more stable viscosity index in extreme temperatures, which is why the US 
Army likes silicone fluids. Since few of us ride at temperatures very much 
below freezing, let alone at 40 below zero, silicone's low temperature 
advantage won't be apparent. Neither fluids will reduce stopping distances. 
With the advent of ABS systems, the limitations of existing brake fluids 
have been recognized and the brake fluid manufacturers have been working on 
formulations with enhanced properties. However, the chosen direction has not 
been silicone. The only major user of silicone is the US Army. It has 
recently asked the SAE about a procedure for converting from silicon back to 
DOT 3-4. If they ever decide to switch, silicone brake fluid will go the way 
of leaded gas. 
Brake system contamination 
The single most common brake system failure caused by a contaminant is 
swelling of the rubber components (piston seals etc.) due to the 
introduction of petroleum based products (motor oil, power steering fluid, 
mineral oil etc.) A small amount is enough to do major damage. Flushing with 
mineral spirits is enough to cause a complete system failure in a short time 
I suspect this is what has happened when some BMW owners changed to DOT 5 
(and then assumed that silicone caused the problem). Flushing with alcohol 
also causes problems. BMW brake systems should be flushed only with DOT 3 or 
4. 
If silicone is introduced into an older brake system, the silicone will 
latch unto the sludge generated by gradual component deterioration and 
create a gelatin like goop which will attract more crud and eventually plug 
up metering orifices or cause pistons to stick. If you have already changed 
to DOT 5, don't compound your initial mistake and change back. Silicone is 
very tenacious stuff and you will never get it all out of your system. Just 
change the fluid regularly. For those who race using silicone fluid, I 
recommend that you crack the bleed screws before each racing session to 
insure that there is no water in the calipers. 
New developments 
Since DOT 4 fluids were developed, it was recognized that borate ester based 
fluids offered the potential for boiling points beyond the 446F requirement, 
thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation - 
which exhibit a minimum dry boiling point of 500F (same as silicone, but 
different chemistry). 
Additionally, a new fluid type based on silicon ester chemistry (not the 
same as silicon) has been developed that exhibits a minimum dry boiling 
point of 590F. It is miscible with DOT 3-4 fluids but has yet to see 
commercial usage. 

    
Herb 

1959 Coronet 326 Poly 
1963 Fury 2D/HT 6.1 
1963 Sport Fury Convertible 361 
1970 Challenger RT 440 
1999 Durango SLT 5.9 
2006 300-C Heritage 5.7 
2008 SRT-8 Magnum 6.1 
St. Louis, MO. 


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