Capt William

I remember when silicone brake fluid came out and it was touted as the greatest advance in brake fluid ever. It is still sold and the specs seem much better then any other type of brake fluid (boiling point, water absorbtion, not to mention not hurting paint or rubber).

I put silicone brake fluid in my TR-8 about 22 years ago, when I upgraded to 4 piston calipers and vented disks, and have never changed it. I have never had to change a hose or rebuild a caliper or cylinder.

It seems that nobody uses silicone brake fluid in any high performance applications any more but I have never heard of a scientific explaination of why. Can anyone explain, specifically, what the problems are with silicone brake fluid?

By the way, I noticed that many of the autoparts stores ore now selling silicone brake fluid (and it's still very expensive) but it says "not for ABS braking system" on the bottle, why?

Seattle Scott

Off the top of my head..it doesn't absorb water so you can get pockets of ice in the lines. Search.

Capt William

I addition, if it doesn't absorb water how do you get pockets of ice in the lines? Where does the water come from if not through the fluid?

Epicman1

It absorbs air much easier than standard fluid, which makes bleeding the brakes more difficult.

ryoung

1. Dot 5 silicone brake fluid is more compressable than glycols, and it becomes more compressable with increasing temperature. The result is a softer brake pedal.

2. Silicone brake fluids are not hygroscopic. Any moisture that condenses in the system (for example, from the air space in the brake fluid reservoir) collects as water drops in the lowest point in the system. The water can freeze in the winter or boil under hard braking.

3. I've heard that silicone brake fluids can foam in ABS pumps.

The advantage of silicone brake fluids: They remain inert and non-corrosive. Glycols get corrosive as they absorb water, and the brake fluid becomes corrosive, damaging master cylinders and brake calipers.

6sidua

but there are some things to consider....
silicon brake fluid (dot 5) has a higher boiling point, that's good but because it doesn't absorb water, if any water enters your system (from humid air or diffusion through rubber hoses) the water will boil during heavy braking and the steam can oxidize calipers or other bare metal components. also any water that accumulates can settle in low spots in the system and again cause oxidation. i think this issue is over played.
dot 3 or 4 or 5.1 (glycol based) fluid will absorb any water, still work and avoid this issue...of course because glycol based fluids do absorb water this means their performance degrades over time with lower boiling points.

dot 5 fluids don't eat paint so they are a good choice for show cars and can be left unchanged for very looooooong periods.

dot 5 fluids are more compresable so brakes feel spongy at all temps. this makes them a poor choice for race cars or abs cars because the abs may not be activated or activation requires greater pedal pressure.

probably the biggest issue with dot 5 is that it does not mix at all with other brake fluids...so getting an absolutely complete flush prior to using silicon is a problem. there cannot be even a drop of any glcol fluids in the system or the mix will "gel" and clog the line.

i think for vintage cars, show cars and car with complete rebuilds silicon brake fluid is fine. it's even fine for track cars once the drive adjusts to the very different feel.

probably why it works fine in your t....
it doesn't really have much braking power anyway...
cheers

Seattle Scott

when I bleed the brakes on my TR-8 the fluid looks perfect. Good for you. On my 97 A6 the first 1/5 of the fluid looked a lot darker and uglier than the new fluid (flushed the fluid about 4/2000). The resevoir fluid was darker than the new fluid (DOT 4). On the other hand, my parents have never touched the brakes on their 92 Audi 80. The fluid is dark but it stops fine.

Capt William

At least not at pressures likely to encountered in a brake system. For solids or liquids to be compressable you normally have to go to many thousands of BAR. These kinds of pressures are used in nuclear weapons to compress fissionable materials and create a greater than critical mass but it requires the shock wave of special explosives to get there, not the thousand of so PSI in a braking system.

The foaming issue seems to be a possible downside. Higher viscosity and so slower responce seems reasonable. I'm not sure I understand the non-hygroscopic nature of silicone fluid causing moisture problems.

I can only say that on my TR-8 with aftermarket 4 piston calipers and heavy duty vented rotors there has been no issues with corrosion, freezing, etc. but I have noticed the brakes fading sometimes with surprisingly little use. On the other hand I have never changed the fluid or bled the brakes since they were installed over 20 years ago.

Capt William

Never added any fluid at all, never replaced any parts (seals, hoses, etc.) either. My point is that on other cars that I have had where I neglected the brake system even for a few years I would get corosion in the calipers and master cylinders that would cause fluid leaks or piston sticking. I was just wondering if there was some way I could get the advantages of silicone without sacrificing some other important braking characteristic.

ryoung

At 400 oF and 1200 psi, compressibility is 0.5% for glycol fluids and 2.4% for silicone brake fluids. The link shows pedal travel increasing ~1/4" with silicone brake fluid and light braking (cool calipers) to 1" or more with heavy braking and hot calipers. This is the main reason DOT 5 fluids are not recommended for track use.<ul><li><a href="http://www.buckeyetriumphs.org/technical/Brakes/Fluid/Fluid.htm">Selecting brake fluid</a></li></ul>