Amazoning with Silicon Brake Fluid

Tech Article - R. Kwas 

This article is likely never to be complete....I intend to add to it as I gather more notes on the subject....naturally, the information presented here is not limited to Amazons, but also applies to its Volvo contemporaries.

First of all:  DOT5.1 is NOT a further development of the Silicon DOT5 fluid as the nomenclature would suggest.  It is in fact of the same formulation and so compatible with DOT4, and not DOT5.  Do not mix the two!  You can thank the idiots at the DOT for that!

Pros-Big Pros Bolded:

Non-hygroscopic (non-moisture absorbing), no decrease in boiling point with moisture absorption therefore no deterioration over time, no internal corrosion of brake components due to fluid moisture content, no need to periodically change fluid.  

Excellent seal/dust boot compatibility, increased seal life/time between rebuilds by 3 times (conservative estimate from first-hand experience with multiple vehicles), no frozen wheel cylinders or empty reservoirs after prolonged vehicle storage.   Independently conducted fluid compatibility test (under UV) with Girling seals yielded very good results.  

Inert.  Non paint-attacking in case of spills.  An nicely painted engine compartment will stay nicely painted - especially under the fluid reservoirs.


Cons-Big Cons Bolded:

More compressible at elevated temps, therefore unsuitable for racing.  When silicon fluid first became available, it was heralded as the grooviest thing for all vehicles since a racers, in their never-ending quest for an advantage tried it and found that at elevated temperatures (a condition they certainly frequently encounter) they would get a soft pedal, making threshold braking difficult if not impossible.  Soooo...Silicon is not suitable for racers or those planning to drive the Mount Washington Auto Road (especially the down trip).

Longer retention of suspended air especially micro-aeration.  This will result in a soft pedal and make repeated system bleeding necessary.  Care must be taken during fluid handling to minimize aeration.  Micro-aeration can occur in rear brake valve during fluid change-over, necessitating repeated system bleeding .

Non-compatible/mixable with other (DOT3/4) fluids.  System must be flushed for fluid change-over.  Top-up fluid should be kept on-board as it may not readily available or be expensive, top-up container should be kept absolutely full to prevent road-vibration caused fluid aeration.  Reservoirs must be clearly marked to prevent topping up with incorrect/incompatible fluid.


Expensive, typical cost is 2-3 times that of DOT4 fluid, but as it does not need to be replaced it breaks even (if not better) over the long haul.  Upon last inquiry, 16oz. were quoted at $8USD (had to be ordered, 2-3 day wait).


Unknowns -To Be Researched:

1.  Affects on motor if ingested and combusted (due for instance to failure of brake vacuum assist unit diaphragm).


Current Sources: 

Few parts houses stock it on shelves, but most can still easily get it on special order.  Eastwood, the restoration specialty supplier out of Pennsylvania used to carry it, but upon last check had discontinued it.  Pooooo


Reservoirs clearly marked to prevent topping up with  incorrect/incompatible fluid.


Notes, and my wiseass comments highlighted in yellow.

DOT Designations and Boiling Points Requirements ( Fahrenheit)

Designation / Formulation New, "Dry fluid" (no moisture content) Used, "Wet fluid" (exposed to moisture)
DOT 3 / Polyglycolether Fluid



DOT 4 / Polyglycolether Fluid



DOT 5.1 / Polyglycolether Fluid



DOT 5 / (The ONLY Silicon Fluid)


356 (unlikely condition since non-hygroscopic)


Minimum wet
boiling point





Just exactly from where is your leg losing brake fluid?

By "bypassing", one presumes the booster was disconnected from the manifold and brake system, so its essentially ballast (which IMO, with their track record, it was before it was installed)...recycle that thing!...rebuilding is a total waste of time and resources.

If there's no leak, it's a bypassing master cylinder. But if there's a leak, as you state, that's a good indication of what corner to start on. It sounds like it's time for some brake work. Components are available no problem...I suggest you replace both sides at a time (and I suggest a few other things too, see:

...normally I don't like to do a whole lot more than absolutely necessary, but on a single circuit brake system, I can certainly justify it...and I suggest you do it real soon, before your brakes fail altogether and you soil yourself for real!

Don't mix brake fluids...that's always the big rule, and you broke it...who knows what exotic reaction may have taken place, and how your seals liked it (or maybe not!). While you're into the brake system, it may be a good time to upgrade. George and I prefer silicon DOT5 fluid. Beware! The new DOT5.1 specification fluid is NOT is a further development of the glycol based DOT4 fluid...thanks the DOT braniacs for that misleading nomenclature (logic, what logic!). See also:
or do a search on Silicon brake fluid...there's lots of good reading out there.
My notes are at:

Laundromats are a highly regionalized phenomenon...I suggest you "let your fingers do the walking", to find one, but its probably better to fix your brakes before you need one.



Info from surfing:

Brake fluid boiling point is a very good comparison for evaluating regular or polyglycolether based fluids, "glycol" for short,  because it roughly represents the conditions under which glycol turns compressible. On the other hand, it is a very poor comparison for evaluating silicone based fluids because they turn compressible at a lower temperature than that at which they boil.

To add to the confusion, (presuming a properly bled system with negligible air bubbles), non-silicon fluids get compressible only after they boil (with absorbed moisture lowering this temperature), however the silicon fluid is four times more compressible at elevated temps (but before boiling point is reached).  This is also not helped by the susceptibility micro-aeration.  [This is the crux of the problem when discussing the fluid options.]

Glycol based brake fluid is hygroscopic, i.e., it absorbs water from the surrounding atmosphere, and it starts adsorbing water as soon as you put it into your car's brake system. The brake fluid reservoirs on top of the master cylinder are vented to the atmosphere. Moisture can also enter the brake system through the rubber seals and hoses through a 'diffusion' process 

From: Chicagoland MG Club techtips [

Silicone Brake Fluid: Snake Oil, It's Not

There have been many views put forth about this product, but two are inescapable: it must work as brake fluid or it never would have received DOT 5 approval, and, it physically cannot absorb moisture. This latter point was confirmed in a test performed by two members of the Society of Automotive Engineers, with the results published in SAE Paper #780661. [No luck on Search for this...yet!] They reported "After two years of service and 56,295 miles, the physical properties of the (silicone brake) fluid were unchanged, and the water content was 0.00%. System wear and corrosion were non-existent."


The Real Story:
Conventional vs. Silicone Brake Fluid


Additional notes (from email):

Bmustnilh@XXXXXXXX wrote:

On your tech article on DOT 5--- I've been trying to find out if DOT 5 is lighter or heavier than water. If lighter there is another big problem. 

Water will still condense in the reservoir with temp and air pressure changes such as the cycling of engine compartment temp. Will such condensation float on top of the DOT 5 or will it drop to the bottom of the reservoir and be distributed through the system defeating the main purpose of DOT 5. 

Please forward to the tech writer.

 My Response:


Thanks for your clearly have an excellent point. Depending on relative densities, if any condensed water were to sink to the bottom, it would certainly get pumped throughout the system and its components and there goes any advantage! I put on my tech writers hat, went out to the garage, and put a couple of drops of water into a small cup of DOT5 fluid...they promptly FLOATED, so I think were OK. 

BTW, as I recall from flushing out DOT4 fluid systems (I like to pump a full reservoir worth through each corner)...with the price of DOT5 fluid being what it is, I typically store the full (glass) drain/bleed bottle on a shelf for a year or so in which time the DOT4 and DOT5 fluids separate out perfectly (DOT5 on top). This allows me to pour the silicon fluid off the top and use it again (for flushing ONLY)...naturally the final reservoir fill is done with fresh fluid.

...any other thoughts, don't hesitate!

Regards, Ron

from Brickboard regarding reassembling split calipers:

Jim McDonald wrote: 

> No sweat; I've done it several times. 
> The only problem is that the o-rings between the halves (you'll see 
> the grooves) aren't available as replacement parts; you'll have to get 
> them from a bearing supply house. The recommended compound for brake 
> fluid is EPR, not neoprene or Viton. 
> Instead of worrying about the torque, use a dab of blue Loctite and 
> tighten the bolts good and tight. Otherwise, look for a torque spec 
> for a similar sized bolt (and still use Loctite). The rest of the 
> assembly procedure is pretty straight forward, but I recommend 
> stainless pistons, from John Farrell Auto Parts. The 122 calipers are 
> the same as 1800S (and XKE 4.2 liter).


I have some split calipers that I've been holding on to...I haven't needed them yet, but have thought about what I would do if I ever needed to put them back into service. Sealing between the two halves is clearly the critical issue...if compatible o-rings are available, that's great (and thanks for the info), but I was thinking that an alternate solution would be to counterbore one of the caliper halves for an interference fit copper squish washer which would certainly compatible with ANY brake fluid. 

Thanks also for the info on Stainless pistons...they can't be cheap!...but I love that stuff!!!



One of the best compilation of fluid characteristics comparison I've run across:

Copied from:





General Recommendations:

  1. If you have a brake system that doesn't leak or show any other signs of failure, but has old seals in it, don't change fluid types as a result of reading this article. If it isn't broken, don't "fix" it -- you may simply break it instead!
  2. Flushing of the brake system every couple years to remove any absorbed or collected water is probably a good idea to prevent corrosion, regardless of the type of brake fluid used.
  3. DOT3 is dangerous to use in Triumphs with natural rubber seals, and thus should not be used in such cars, except as a temporary "quick fix to get me home" solution. (If this is used as a "get-me-home" solution, bleed the system as soon as possible, and be prepared to replace all your seals.)
  4. DOT3 is an adequate brake fluid for use in later Triumphs, although it is rarely preferred. My recommendation would be to simply not use it.
  5. DOT4 fluid, for a slight increase in cost, will give significantly increased resistance to moisture absorption, thus decreasing the likelihood of corrosion compared to DOT3.
  6. DOT4 fluid has a higher boiling point than DOT3, making it preferable for high performance uses such as racing, autocross, or excessive use of the brakes in mountainous areas. For even greater braking performance, consider going to DOT5.1 or a high-performance version of DOT4 fluid.
  7. DOT5 is a good choice for the weekend driver/show car. It doesn't absorb water and it doesn't eat paint. One caveat is that because it doesn't absorb water, water that gets in the system will tend to collect at low points. In this scenario, it would actually be promoting corrosion!
  8. DOT5 is probably not the thing to use in your race car although it is rated to stand up to the heat generated during racing conditions. The reason for this recommendation is the difficult bleeding mentioned above.
  9. When changing from one fluid type to another, as a minimum, bleed all of the old fluid out of the system completely. For best results, all the seals in the system should be replaced.
  10. As always, your experiences may vary.  [obvious the guy who wrote this a lawyer?]



[From]... posted by Ron Kwas on Tue, May 6th 2003 at 9:23 AM response to "Opinions on DOT 5.1? [120-130]" posted by JohnMc  on Mon, May 5th 2003 at 11:42 AM 

Link to Thread:


First I'll state that I have no first hand experience with this fluid, so all I can do is just opine on what I know...and that's what you did ask for!

DOT 5.1 does seem to be a good solution if you just want to replace the fluid and don't want to otherwise open up the system...the makers have addressed the most important issues (compatibility with polyglycolether fluids, moisture neutralization, seal compatibility is naturally paramount and so presumed) would seem to be an upgrade in both temperature performance and long term corrosion because of the moisture neutralizing characteristics, and doable by just replacing and bleeding since its fully compatible with the old fluid. So far so good.

The blanket statement of "10 years" bothers me a bit though...I would expect the fluid moisture neutralizing mechanism to to be limited to some finite amount (its typically some compound which will grab and bind-up any water molecules, keeping them from doing the same to ferrous components of the system), and they do not state what happens after that mechanism gets saturated and used up (moisture separation, and the accompanying BP decrease etc.)...the point is that "10 years" is probably the sales department talking of the bestcase (Arizona?) where very little moisture is absorbed. So it would seem that the fluid still needs periodic, scheduled replacement.

So it would seem to be some improvement over the fluids which had no moisture absorbing mechanism, but I would love to hear from someone about the longterm performance (without replacement, and in a slightly more typical moisture environment - like New England, or the rainy pacific NW). The question I have which I guess I would love to see an answer to is: What increases the hydraulic system life so much when silicon fluid is used in comparison to polyglycolether it the non moisture absorbing characteristic, or the fluid/seal combination itself...I guess now we can find soon as someone has the fluid in a system for longer that 5 or 6 years...




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