The GM stuff is DOT 3, Ford (used it in my C5) is also DOT 3 but has a much higher dry boiling point, wet is similar to GM. I'd like to switch to a DOT 4 with the higher boiling points, especially the wet boiling point. They are all SUPPOSED to be compatible with each other, and suitable for normal use in a daily driver. But I'm always suspicious- are there any problems using any of the DOT 4 fluids with once-per-year flush? I don't do road courses in this car, just autoX and enthusiastic street driving.

 

There is a very good article about brke fluids in either Corvette enthousiat or Vette isue of this month...if you don't get an asnswer by tonight I will dig it for you when I am back home

 

Someplace in the past I saw a test on various DOT3 and 4 fluids, but don't remember where.

The think that stuck in my memory was that the highest boiling point wasn't always the best for street, because some of the highest boiling point fluids were worse at sucking moisture out of the air.

If anyone can find that article it would be nice to have posted.

Edit: Spent about 15 minutes Googling for the article and couldn't find it. Did find some comments about the desirability of low absorbtion rates, but no real numbers.

 

DOT 4 is fine - just avoid DOT5 as it's silicone based (DOT 3 and 4 are glycol-based). DOT 5 will not absorb water which is bad for a road car (Yes I know there is a 5.1 spec which is glycol-based - I'm just trying to answer the question).

So bottom line is use DOT 4 and make sure it's from a fresh bottle. You'll be fine.

 

Don't you just love the DOT 5 - DOT 5.1 naming sequence?

 

Dot 3 is natural glycols with specific dry and wet boiling point minimums
Dot 4 is natural or synthetic glycols with specific dry and wet boiling point minimums (that are significantly higher than Dot 3 minimums)
Dot 5 is synthetic silicone
Dot 5.1 is synthetic glycols with the same specific boil point minimums as Dot 5 silicone stuff.

Glycol is a fluid closely related to alcohol, and like alcohol (200% pure) it will suck water out of the surrounding air even in the Namib desert in the middle of a summer day (5% humidity 115dF). The seals in the brake fluid system are primarily designed to keep the water out (or slow it down to a minimum) even more than they are designed to keep the brake fluid in.

A fresh flush of perfectly dry Dot 4 will have the dry boiling point characteristics in Phoenix durring the summer of just over 1 week, and by 6 months you will be operating near the wet boiling point levels. More humid climate will allow the fluid to reach its wet boiling pint even faster. Since glycol is so efficient at sucking water vapor right out of the air, it is a very efficient fluid at moving water throughout the brake system. Within a few hours of flushing with dry fluid, the water vapor sucked in near the brake seals will reach the brake reservoir. Bleeding the brakes only gets the fluid exposed to the heat of the brakes doing their job, and does nothing for the corrosional aspects of the fluid in the system en-the-large.

So, for cars driven on the street, you should change (e.g flush) the fluid about once a year this will protect the internals of the brake system from the corrosional damage that water can do to a system designed for dry glycol operatioin.

Dot 3, Dot 4, and Dot 5.1 can be mixed with no ill effects (like turning from a fluid into an emulsion.) You will likely end up with the wet boilng point near the wet boilng point of the fluid with the lowest numbers. So there is no advantage of filling up a Dot 3 system with Dot 4. If you want a Dot 4 capability, flush the entire system with Dot 4.

Mixing Dot 3, Dot 4, or Dot 5.1 with Dot 5 will cause a sticky highly viscous substance to be created. This will cause you to have to flush the entire system with denatured alcohol, and blowing air through the lines, disassembling the calipers, and installing new seals, (you may need a new master cylinder) before your brakes are ready to go again. Do not mix anything with Dot 5.

For street driving where you do not expect to disipate more than 150 MPH of energy in a single hard braking event, Dot 3 is sufficient for your needs even when 1 year old and operating at its wet boiling points.

For track driving, Dot 4 less than 1 week old is right on the edge of making you succeptable to problems. You should be looking at dry boiling point in the 500dF range, or a fluid like LMA or SRF that do not suck on the water so hard and last longer.

 

I use Motul 5.1 in every vehicle I own, car or bike. Stuff just flat out works better and lasts longer than the regular chit.

 

That's some pure stuff.

 

Great post... learned more about brake fluid from your post then most long winded articles I've read......

 

Castrol GT/LMA meets or exceeds Dot 3 and Dot 4, I use it for both brake and clutch. One 12oz can fills both master cylinders after a turkey baster fluid removal.

 

There are supposed to be some fluid test trips that you can dip in the reservoir fluid and will give you an indication of how much moisture is in the fluid.

http://www.aa1car.com/library/bfluid.htm

 

Great post! Thanks.

 

One of the tests a brake fluid has to pass for DOT certification is a DOT compatability test so you should not expect any problems to be created by mixing DOT 3 & 4.

What's a "natural glycol?

 

A couple of years back I used (as I recall) Valvoline Syntech Dot 5 brake fluid in my '99, I'm the label said it was Ok to mix with other brake fluids and I didn't notice any problems with the car. I no longer have it but I kept it for another 18 months after doing the switch. I know it was a synthetic so I'm wondering if some mix better and some don't?

 

Wow! MitchAlsup's post was like attending a college lecture. That was great and very informative. Thank you MitchAlsup.

 

I don't have more to add on the DOT 3 vs DOT 4 debate. DOT 4 and DOT 3 are interchangeable so DOT 4 is preferrable because of the higher boiling points.

The main thing I want to emphasize is that I did run into the problems of brake fluids overheating on the track, and lost all braking. And I attribute it to old brake fluid that hasn't been changed for over 2 years. Brake fluid absorbs moisture over time. Make sure to change it regularly.