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View Full Version : OT- Your opinion of DOT5 Silicone Brake Fluid



Doozer
11-30-2012, 10:28 PM
I have 2 old trucks.
One is a '53 IHC pickup.
About 6 years ago, I I replaced all the brake hoses and
the master and wheel cylinders. All the rubber stuff.
I converted it to DOT5 because of all the nasty corrosion
in the master and wheel cylinders. And because I put in
all new rubber components, allowing to convert without
fear of mixing and contaminating the non-compatible
fluids. The steel lines were flushed with alcohol and
blown out with compressed air.
There have been no problems. I feel that corrosion
will never be a problem again in that brake system.

Now I bought a '68 IHC 5-ton truck, with drum brakes.
I was thinking of replacing hoses and master and wheel
cylinders just like I did with my pickup truck. Just for
fun I googled DOT5 and found people with various
problems. Some stemming from not flushing their
systems well enough, and some used it on race cars
with disc brakes, where heat can boil the fluid easier.
Perhaps only the bad gets posted about DOT5 and
when it works well, no one says anything about it, as
when something works, it just is ignored and taken
for granted.

Anyone here had good or bad things to say about
DOT5 fluid?

Also, since this is drum brakes and the fluid would see
little heat, why not use thin hydraulic oil??
I think some fork lifts and military trucks use hydraulic oil
in the brakes. Any one can comment on this?

Thanks,
Doozer

Mike Folks
11-30-2012, 10:39 PM
Some information:

Brake Fluid Problems

"I like to go with DOT 5 fluid when I rebuild the brakes, just a better product"/quote]

Maybe not better, but certainly different. An alarm went off in my brain, so I Googled this up.

"More than you ever wanted to know about brake fluid....

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:

Fluid compatibility with the brake system rubber, plastic and metal components.

Water absorption and corrosion.
* Fluid boiling point and other physical.
* 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. DOT4 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. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. 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 ( 446 F), 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 side 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 car owners changed to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. Older 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 fluds but has yet to see commercial usage."

I learned in the past thru others mistakes not to switch to DOT 5 in systems designed for DOT 3 or 4.

From the above article, I gather you can't go back to DOT 3 or 4 if you've used DOT 5 brake fluid.

HWooldridge
11-30-2012, 10:53 PM
DOT 5 is used in a lot of vintage vehicles because it's supposedly not hygroscopic and can be left sitting for long periods. I have a gallon that's been in storage for about ten years that I keep threatening in my old '61 Dodge but have never made the conversion.

topct
11-30-2012, 11:28 PM
It is not a simple conversion. The old system must be completely free (as in like new) of the of non silicon fluid. "ALL" rubber components must be also be compatible with Dot 5.

Period.

browne92
12-01-2012, 09:41 AM
Also, since this is drum brakes and the fluid would see
little heat, why not use thin hydraulic oil??
I think some fork lifts and military trucks use hydraulic oil
in the brakes. Any one can comment on this?

Thanks,
Doozer

I wouldn't try this simply because if you were to get into any kind of accident, your fault or not, the lawyers would have a field day with it.

A.K. Boomer
12-01-2012, 10:15 AM
Also, since this is drum brakes and the fluid would see
little heat, why not use thin hydraulic oil??
I think some fork lifts and military trucks use hydraulic oil
in the brakes. Any one can comment on this?

Thanks,
Doozer




Im in agreement with Brown and would not do this at all - for one I don't believe your seals are designed for oil - they are designed for brake fluid - which is not oil - water washes brake fluid away,

I think your seals would swell,
Personally I don't mess with success - I just use the standard dot 3or4,
Iv seen cars with close to a half million miles on them and doing fine even though the stuff is dark and original --- if they can go that kind of miles just think what they could do if not neglected,
all it takes is a flush once in while and your good to go for a long long time...

incidentally - both brake fluid and power steering fluid are about the two most ignored things on a vehicle - usually people don't do anything with them till it's too late, then they blame the components for failing...

vpt
12-01-2012, 10:19 AM
I just run normal dot3/4 in everything. The race cars get blue stuff.

john hobdeclipe
12-01-2012, 10:31 AM
Would someone please explain this paragraph to me?



Fluid boiling point
DOT 4 glycol based fluid has a higher boiling point ( 446F) than DOT 3 ( 446 F), 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 side 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.

krutch
12-01-2012, 01:27 PM
Post #2 is more info than I've read before. Thanks!
I have known for some time you can use dot 5 in a 3 or 4 system if the system was new or completely clean, however dot 3 or 4 will not work with dot 5 rubber componets. 3 & 4 will "eat" the dot 5 rubber. Mixing is a NO NO, you wind up with jelly, at best.

RWO
12-01-2012, 01:45 PM
I bought a new 86 Nissan Maxima. The next day I drained the brakes of all the fluid that would run out by gravity and cleaned out the master cylinder reservoir. I then filled it with DOT 5 Silicone and bled the system generously. 10 years and 40,000 miles later I flushed the sustem and put in new DOT 5. 10 more years and 35,000 miles later, I sold the car. The only brake work ever done was to change pads. It never leaked a drop, brake performance was excellent and never varied. Maybe I was lucky.

RWO

Void
12-01-2012, 05:13 PM
Wikipedia has a pretty good page on brake fluid:
http://en.wikipedia.org/wiki/Brake_fluid

Back a few careers ago when I worked on cars for a living (1980s)... I found that silicone/DOT 5 did not work well in ALL brake systems but it was fine in most. Sometimes even if the seals are compatible DOT5 just won't work well. Might be due to slight differences in viscosity.
I have, at one time or another, seen most of the problems associated with DOT3 and 4 fluids.

Unless the system was designed for it I would discourage experimenting with DOT5 in ABS brake systems.

Other than the aforementioned problems I found DOT5 to be superior to the others in every way. On my customers cars I was careful to affix a tag to the MC cover that said "This system uses DOT5 brake fluid only."

-DU-

Don Young
12-01-2012, 09:26 PM
Would someone please explain this paragraph to me?

I think it has two typos. I do not know which '446" is correct but it should be easy to search and find out. The 'would side at' seems more like 'would solidify at'.

RandyZ
12-02-2012, 11:44 AM
I've been using silicone brake fluid in my 51 mercury pickup for 20 years now. not one bit of problems with the hoses or the wheel cylinders. The big problem I have is with the brake light pressure switches. They only last about a year or two and then the silicone weeps past the diaphragm and contaminates the contacts, resulting in no brake lights. I had heard that some of the Harley bikes used a special switch designed for silicone. It lasted about 4 years and did the same thing. I've resorted to using a mechanical switch off the brake pedal.

Willy
12-02-2012, 02:50 PM
I had heard that some of the Harley bikes used a special switch designed for silicone. It lasted about 4 years and did the same thing.

It's been in my HD for about 35 years without issues.
The US military has standardized the use of it for everything since the early 90s I believe.
Perhaps it is the make or brand of switch causing the problem, the switch may not be compatible with silicone based fluids.

Tuckerfan
12-02-2012, 04:34 PM
I had a leaky brake line that I couldn't pin down for a while, and the system would lose enough brake fluid that the warning light would come on every couple of months, and then I'd have to add more to the system. I mistakenly grabbed a bottle of DOT 5 fluid and dumped it into the reservoir one day. I didn't notice any issues with the brakes themselves, but when the fluid ran low again, the warning light did not come on, even though the pedal was dangerously soft.

Shortly after that happened, I found the source of the leak, fixed it, and drained the DOT 5 stuff out and filled it back up with DOT 3. I would suspect that the real issue is not so much the fluid used, as the braking system. Some of them are probably designed with characteristics (unintentionally so) which cause issues when DOT 5 is put in them.

Black_Moons
12-02-2012, 04:50 PM
I like the idea of dot3/4 more myself.
Yes, it can take on water from the atmosphere, but it keeps it distributed, if you flush your drain lines and put in fresh brake fluid, you know your system is CLEAN. The corrosion inhibitors will handle low water content, so just swap the fluid reasonabley regularly.

Dot5 as mentioned will leave globulas of water in your system that may or may not come out while flushed and will settle down to the lowest point in your system and will start corroding through.
Sure, DOT5 boils higher, but if you are boiling your brake fluid you are doing something wrong anyway.

RWO
12-03-2012, 09:29 AM
Often hear the comment regarding the inability of silicone fluid to absorb water like the Dot 3 fluid does. Dot 3 fluid is hydrophylic by design and will absorb water through the hoses according to what I have read. Silicone fliud is hydrophobic and thus has no tendency to absorb water. So how does water enter a braking system filled with silicone fluid? The only place where the fluid would be exposed to air would be the surface of the fluid in the master cylinder resorvoir and modern resorvoirs have a rubber diaphragm seal under the cap. I have asked this question to several "experts" and have never gotten a good answer.

Regarding use of silicone fluid in ABS systems, the high viscosity and slight compressability of silicone fluid are alledged to be the problems.

RWO

Wirecutter
12-03-2012, 01:26 PM
I have made a few electric gokarts, and they all use DOT 5 (silicone based) brake fluid. Okay, okay, it's not quite as critical an application as the brakes on a school bus or a tanker truck. :D :D

The biggest difference to a simpleton such as myself is that the more traditional DOT 3 (and DOT 4, perhaps?) fluids will peel paint. DOT 5, if allowed on any unpainted surface, is nearly impossible to clean off completely and will prevent paint from sticking to the surface.

-Mark

Rex
12-03-2012, 04:46 PM
Years ago I rebuilt the brake system on a '66 Mustang with manual disks. All new components, and then-new DOT5 fluid. Hardest brake pedal I've ever had. This was my wife's daily driver for a while. The wife's right leg muscled up pretty good, I had to start being nice to her.

Shortly after silicone fluid came out SCCA banned DOT5 from race cars, because it expanded when hot and you had a mushy pedal right when you needed brakes the most.

When you mix DOT5 silicone fluid with DOT3, you get gray mud.
About the same time some of the new ABS systems (Chrysler) came out with very complex master cylinders with tiny orifices. That gray mud would plug those orifices real quick. That's about when the DOT5 excitement was over.

I don't use DOT5 any more, but I would if I had an older car that doesn't get used much.

Which reminds me, I have 5 vehicles that need the fluid changed.