driver9

According to the 2016 owner's manual the specification for brake fluid is "DOT 3 hydraulic brake fluid" followed by some GM part numbers. The specification for clutch fluid is "Hydraulic clutch fluid" followed by some GM part numbers then "Super DOT4 brake fluid."

It seems odd to me that they chose a different fluid for the brakes and the clutch, especially because the clutch fluid is a much better fluid than the brake fluid. It absorbs water less easily and has a much higher boiling point. It seems counterintuitive to use a higher spec fluid on the clutch since it would seem like the brakes would get much hotter than the clutch. The clutch hydraulics actuate a slave cylinder that intermittently comes in contact with the throwout bearing. The throwout bearing would seem to be a lot less of a heat source than brake pads whose job it is to convert kinetic energy into heat by frictioning against the brake rotors. The brake fluid is directly part of the thermal chain that connects to the brake pads and thus would be expected to get very hot.

I must be missing something. Does anybody have an idea why the specification is DOT 3 for brakes and Super DOT 4 for the clutch?

Ernest_T

Different engineers? Dot 4 is compatible with Dot 3 so I'd 4. Did you look up the GM part number, it may be Dot 4?

Gearhead Jim

I've seen other commentary and charts saying that DOT4 fluid absorbs moisture from the air more rapidly than DOT3. The result would be that the performance (boiling point) advantage of DOT4 would decrease over time and approach DOT3 at some point. And the extra moisture in the DOT4 fluid would promote more corrosion in the brake system.

Doesn't the Manual also say something about using higher temp brake fluid for track use?

driver9

It says to use "qualified high performance" brake fluid. And it says that brake fluid with a dry boiling point > 279 degrees C (534 F) is "qualified".

Oddly it does not look like it's possible to meet that specification. Pentosin Super DOT 4 shows a dry boiling point of > 265 C / 509 F and that's far above DOT 3 and DOT 4 specs.


The owner's manual also says "If high performance brake fluid is used, replace it with GM approved brake fluid before driving on public roads." The "If" seems to imply that maybe you won't be able to find or use "high performance" brake fluid. The "replace it with GM approved brake fluid" part seems to imply that "high performance" brake fluid isn't GM approved.

This needs to be filed under "you can't win" or "Catch 22".


Edit: It looks like you are right about DOT 4 absorbing water more quickly. Not sure where I read the opposite. Thanks for the correction.


Oh and Ernest_T, per your suggestion I checked the part number and it does look like GMs part number is for a DOT 3 fluid.

spearfish25

By the time the small amount of moisture absorbed by DOT4 brake fluid causes corrosion in my brake system, someone else will be the owner of my car.

TEXHAWK0

Remember, the clutch lines usually run beside the exhaust manifold and can pick up a lot of engine heat there.
On brake fluid, I would be more worried about the fluid attracting moisture and lowering the boiling point.

Gary '09 C6

Use DOT 4 for both, and flush/bleed both every two yrs (more often if regularly tracking the car).

driver9

Interesting point about the exhaust manifold. I'll look at the routing of the clutch lines next time I have the car on a lift.

driver9

That seems like the right idea but there are some voices out there that claim you should do some kind of special flush (e.g. with pure isopropyl alcohol) prior to changing between DOT 3 and DOT 4. Doesn't quite make sense to me, since DOT 3 and DOT 4 are supposed to be compatible.

JerryU

The trade-off is high temp performance versus moisture absorption. DOT 4 if fine for the clutch since I use the Ranger Method every year and refill ~6 times with 30 pumps each fill. That replaces 99% of the DOT 4. For brakes the manual says replace every 3 years, much more of a project.

Racing brake fluid has even higher temp performance but also more "hygroscopic" and the reason it's recommended to remove it after racing.

I put DOT 5 in my ProStreet Rod since TCI (the chassis builder) supplied two quarts. Big mistake as it created a spongy peddle! TCI said it was supplied since it did not dissolve paint as does DOT 3/4 and it is not hygroscopic! Although my chassis is painted I switched to DOT 4! Spongy peddle was gone! Changing from DOT 5 does require purging with alcohol as DOT 5 is NOT compatible with DOT 3 or 4!

The key reason DOT 5 cannot be used in modern cars is the small compressibility that makes it feel spongy also creates problems with the anti-lock braking system. That system quickly modulates the brakes and DOT 5 does not react sufficiently quickly.

JerryU

Hmm, not so sure. This is some info from a good article about brake fluids:
How fast does brake fluid absorb moisture?
It depends on both the fluid and the environment. A typical high performance DOT 4 fluid in a high- humidity environment will absorb as much as 4.5-5.0% moisture in as short a period as 2 weeks if not kept in a tightly sealed container.

That is why you should buy small containers and discard after opening. And also don't open the reseroir covers more than needed. Metal cans do not allow moisture to penetrate into the container. All plastics will allow moisture to enter, it's called permeation! Wonder why GM doesn't use metal for the clutch fluid reservoir? I'll answer my own question, cost!

juanvaldez

Bullshit. Just keep the lid on tightly. Plastic isn't permeable.

Gearhead Jim

It's well accepted that brake fluid in your car does absorb moisture over time, which is why you should flush it occasionally.
The plastic parts may or may not be permeable, but the only thing that counts is that moisture does get in and does get absorbed.

The brake system in your car is a "tightly sealed container", but not perfectly sealed. So the fluid will last much longer than in an open can, but not forever.