Question: Brake Fluid

[Don Mason]

Is there a way to tell whether the brake fluid currently in my system is DOT-3/4 or synthetic? I understand it is not a good idea to mix the two. As always thanks for the good advice I always get and read on the forum.

 

[Vettehead Mikey]

All brake fluid is synthetic. If you mean DOT 5, take a sample of the fluid and mix it in a jar with some water. If the fluids separate after a period of time, it's DOT 5.

 

[kpic]

All DOT 3, 4 & 5.1 fluids are all synthetic. I believe DOT 5 is silicone.

 

[Hib Halverson]

Most DOT3 and 4 fluids are "synthetic". All DOT 5.1 fluid is synthetic. DOT5 fluid is synthetic as well but it's silicon based and should not be used in a high-performance application or in any brake system that has ABS.

If the car is an '01, I wouldn't worry about mixing fluid. I'd change the fluid. Fush the old fluid out of the system, refil with a good DOT4 such as Valvoline Synthetic then bleed per the service manual including the autobleed function for the ABS.

 

[kpic]

Which ones aren't synthetic?

Edit:
http://sosoil.co.za/knowledge-centre/what-are-synthetic-lubricants/

 

[Don Mason]

Which ones aren't synthetic?

Edit:
What are Synthetic Lubricants? - SOS Oil

Sorry, Synthetic/Silicone had my terms confused. Thanks for all your responses and helping me clear that up.

 

[glen242]

Most DOT3 and 4 fluids are "synthetic". All DOT 5.1 fluid is synthetic. DOT5 fluid is synthetic as well but it's silicon based and should not be used in a high-performance application or in any brake system that has ABS.

If the car is an '01, I wouldn't worry about mixing fluid. I'd change the fluid. Fush the old fluid out of the system, refil with a good DOT4 such as Valvoline Synthetic then bleed per the service manual including the autobleed function for the ABS.

What would be considered, in your opinion, a high-performance application?

What reason should DOT 5 not be used in a strictly street and/or weekend drag strip vehicle, such as a C3?

 

[Hib Halverson]

What would be considered, in your opinion, a high-performance application?

1) Very aggressive street driving
2) Racing

What reason should DOT 5 not be used in a strictly street and/or weekend drag strip vehicle, such as a C3?

Silicone fluid compresses when it gets hot so, in an aggressive driving environment, where the brakes are getting used aggressively, pedal feel is not optimal. It gets a bit soft–as if there is a little air in the system.

In my C3, a 71 hot rod, I have never used DOT 5.0 fluid for the above reason.

 

[ZZ1-70 LT4-96]

Dot5

I have used DOT5 silicone brake fluid in my 1969 and 1970 Corvettes for over 25 years now. What amazes me is how clean looking the fluid has remained in the master cylinders, I can actually see the bottom of the brake's master cylinders cavities after all the years of use.

 

[frank_nesta]

Dot 5

I have used dot 5 for 20 to 25 year. What I liked about it when I was not driving my C3 for 3 to 6 month at a time the seals never dried out or leaked. Dot 5 also does not absorb water. Since my restro mod the car gets driven regularly. I drive my car aggressively, not "very aggressive" and no racing. I have never had a problem with soft pedal. As stated above it is amazing how well it holds up. Still clear in the master cylinder. One other reason for my choice is Dot 5 it does not eat paint. I do flush out the system about every 5 years. For my circumstances Dot 5.

Each one of us has his or her own logic for what we do to our cars and taylors it to our set of circumstances. None are wrong just different circumstances. How you use your car will define what you use. Choose accordingly and enjoy your car.

 

[kpic]

My driving style varies from Mr. Pokey to aggressive enjoying the curvy mountain roads.

frank_nesta
I flush it also, but it doesn't ever look as if it needed flushed.

Here is a pretty interesting article: Selecting Brake Fluid All kinds of neat stuff.

"The wet boiling point for the DOT 5 fluid is the DOT specification. The silicone fluid absorbs a very small amount of water (<0.3%) so the boiling point doesn't change with use and one can expect the boiling point to remain at ~ 230 °C/500 °F for many years."

Although when it is new DOT5.1 has a higher boiling point as it is hygroscopic the advantage fades over time. The compressibility tests were performed on Europe's Alpine descents which are quite tasking. We use them also, but for engine tests, not for brake testing. These grades are at an extreme very few of us will ever approach. (But would love the chance )

Edit to add another good read:
http://forums.mtbr.com/brake-time/silicone-based-dot5-busting-myth-832808.html