Synthetic Brake Fluid

[Mel]

Putting new bearings, etc., on the rear axles on my '66. As such, brake backing plates come off and, upon replacement, I bled the cylinders. DOT 3 fluid is much more expensive than synthetic at the local AutoZone -- literally, several times more expensive. I've owned the car about two years and, as I do not know the history of a brake system overhaul, or if there even has been one in the last few years, I'd at least like to flush the entire system. None of the front or rear cylinders leak and brakes are good but the fluid I bled out of the back cylinders was somewhat cloudy. I'd guess that synthetic fluid would not harm a system as long as I flushed the entire system, but I do not know. Does anyone?

[Ray K.]

Putting new bearings, etc., on the rear axles on my '66. As such, brake backing plates come off and, upon replacement, I bled the cylinders. DOT 3 fluid is much more expensive than synthetic at the local AutoZone -- literally, several times more expensive. I've owned the car about two years and, as I do not know the history of a brake system overhaul, or if there even has been one in the last few years, I'd at least like to flush the entire system. None of the front or rear cylinders leak and brakes are good but the fluid I bled out of the back cylinders was somewhat cloudy. I'd guess that synthetic fluid would not harm a system as long as I flushed the entire system, but I do not know. Does anyone?

DOT 3 and DOT 4 are polyglycol-base fluids. Dot 5 is silicone based. Polyglycol and silicone are both synthetic chemicals, and are incompatible. They should not be mixed. The entire brake system must be properly flushed when switching from one to the other. You can tell what kind of fluid you have by placing a few drops into a glass of water. If they dissolve, you have polyglycol; if they separate and sink to the bottom, you have silicone. Also, polyglycol has a distinctive odor, whereas silicone is practically odor-free. My best advise is to stick with the same type fluid that is already in the system when you flush it.

Ray

[Gunslinger]

DO NOT ADD SYNTHETIC FLUID IF THE SYSTEM HAS HAD NON-SYNTHETIC DOT3 OR DOT4! I cannot stress that enough...they are simply not compatible! The cloudiness you see is the result of their being mixed at some point. It could be that the system is contaminated from rust or water rather than synthetic fluid being mixed but assume it has.

The only way you should convert to synthetic fluid is not just flush the entire system, but replace all the hoses, lines, and rebuild the calipers and wheel cylinders to eliminate any vestiges of non-synthetic fluid. Synthetic brake fluid does not absorb water (which is good), but is very difficult to completely bleed of air. Sporting driving can aerate the fluid and the bubbles will form restrictions in the line, resulting in ineffective braking. Most, if not every, performance automobile company specifically says not to use DOT 5 synthetic brake fluid. While synthetic fluid has it's good points and can be used quite successfully, it must be bled absolutely of any air.

If you mix synthetic fluid with non-synthetic, they won't mix and leave a cloudy and murky looking mixture in your brake system and you won't be able to bleed it effectively. It results in a spongy brake pedal and very reduced braking. It will feel like your brakes have faded away. No matter how well you flush it, there can be traces of the old fluid in the rubber hoses, the caliper seals and wheel cylinder cups and seals. Replacing everything is the way to go if you want to convert.

Avanti Motors began using synthetic fluid as factory fill about 1970...at least that's when they started saying so in their sales literature. The sales lit incorrectly referred to it as "silicon" brake fluid rather than silicone. It makes me wonder how many Avanti's have suffered brake problems over the years due to fluids being mixed by owners and techs not aware of the lack of compatibility of the two type of brake fluid. I don't think the industry in general had a good handle on it, much less individual owners and workers in smaller shops. If Avanti Motors stopped using DOT 5 at some point I'm not aware.

I'm surprised that you're finding DOT 3 more expensive than DOT 5...it's usually the opposite. Since you've only owned the car two years, unless you have a service history, you should replace all the rubber brake lines for safety sake, flush the entire system again. Any car's brakes are simply too important and critical to take chances with. Even the best brakes are barely good enough.

BTW - besides DOT3 and DOT4 which are non-synthetic and DOT5 which is synthetic, there's now also a DOT 5.1, which is backwards compatible with 3 & 4. It's confusing and maybe shouldn't be given the DOT 5 in it's rating, but be sure of what you're buying.

There should be some markings on your master cylinder cap or a sticker close by to say which brake fluid to use.

[Paul K.]

This thread did not sound right based on what I have learned and read over the years so I called Castrol Oil this morning and asked them. I have used their GT LMA brake fluid for years and it too has been reformulated and is advertised as a synthetic.

I had a good talk with the service engineer there and spoke of too much to list here but the bottom line on brake fluid (according to Castrol) is that synthetic brake fluid IS NOT silicone brake fluid and IS COMPATIBLE with DOT 3 and 4 fluids.

I also remember silicone brake fluids being a purple color and about $20 or more a qt.

[Gunslinger]

According to Castrol's website, their LMA brake fluid is not specified as a synthetic, though it states it's superior to conventional brake fluids. While it doesn't say so, it may be DOT 5.1 which is compatible with DOT 3 and 4. DOT 5 is considered a silicone brake fluid and not compatible.

Maybe I'm guilty of some wrong terminology, but silicone DOT 5 fluid is a synthetic. DOT 5.1, while also synthetic, is not silicone based.

There's still lots of misinformation and confusion about brake fluids. We all have to be careful what we use.

[BobS]

Synthetic or not by definition doesn't mean as much to me as the ability to adsorb water. The major advantage of Silicon fluid is it is hydrophobic ( won't adsorb water) whereas normal Dot 3 and 4 are hydrophilic (adsorbs water) and that leads to the corrosion and rust in a brake system.

The question should be: Do the Dot 3 and 4 fluids labeled as synthetic adsorb less water than the non-synthetic fluids? If so, then they would a great choice to replace the standard types. Otherwise, take Bruce's advice and just drain and replace what you have with standard Dot 3 or 4. I'd only go to Dot 5 if I was replacing the brake system.

I've used Dot 5 in my 54K, AH 3000 and 39 Ford street rod and like it very much. In all cases, the system was totally replaced. My 83 Avanti is mostly Synthetic Dot 3, only because that was on sale when I removed the rear axle and flushed the front and rear. It stops great,as before, but it would get Dot 5 if the system was completely replaced.

I won't go into the temperature handling ability because I don't think many of we Avanti drivers are going to exceed the temperature ability of Dot 3.

Bob

Edited November 12, 2010 by BobS

[Paul K.]

The LMA container shown on Castrol's website is their old container, the new one can be found if you dig a little. The person at Castrol said that their LMA has always been synthetic it just was not advertised as one. The LM stands for "low moisture" and it absorbs less moisture. Synthetic is not a DOT 5, however because of its raised boiling point it exceeds DOT 3 and 4 specs. DOT 5 (silicone) has no wet boiling point because it does not absorb moisture so any brake fluid with a wet boiling point spec is not a silicone.

Edited November 12, 2010 by Paul K.