If, as I have read from multiple sources, DOT 4 and DOT 3 fluid is interchangeable, why would a vehicle specify 'DOT 3 ONLY'? Is it a materials-compatibility thing or, since this is a GM truck with the stupid intentionally-squidgy pedal feel, simply part of the idiot engineers trying to make sure the pedal is nice and soft to keep the idiot soccer moms happy? Will anything dissolve if I use DOT 4 (or 5.1)?
My thoughts are that I would see the DOT 3 ONLY on vehicles around 2005 to 2006 or older. After that it would be the DOT 4. I think the "ONLY" came into play so dumb-ass's wouldn't put power steering fluid, engine oil, coolant, or washer fluid in it. I have seen all the above. DOT 4 can absorb more water and has a higher boiling point and I agree with you DOT 3 does seem to have more compress-ability.
Well, part of what makes me doubt that they were lazy about it and hadn't made the official call to switch over by that year (2006, in my specific case) is that they kept the DOT 3 specification until the platform ended in 2009. And, looking at various owners manuals, other vehicles that call for DOT 3 in the brakes call for DOT 4 SUPER in the clutch if manual trans, so they were aware it existed and had systems that used it, yet still said 'don't use it in the brakes'.
In random internet articles I have seen it says that 4 is compatible with systems made for 3, except in certain circumstances where there may be materials incompatibility. And of course no one ever gives specific examples of which vehicles are not compatible with the change. Super helpful there, internet!
I get it. 24 years at a GM dealer I know we have seen everything at least once. Hahahaha. The ONLY reason I could think that DOT 3 would be an option is maybe they don't want it to absorb as much moisture. Might last a little longer. Bet you remember when they had the phonelic (sic) pucks in the brake calipers. Maybe that is why? PURE guess on my part.
Or maybe they want you to use the shittiest fluid available to 'maintain original pedal feel' which in my opinion is like stepping on a paper bag of fresh, wet dog turds. Don't get me wrong, the brakes are fine, everything works like it did when new (which was shit even then), but does it really need what feels like 8 inches of pedal travel to make a panic stop?
This truck does have phenolic pistons in the front. Is that an issue with fluid compatibility? I have not seen one single mention anywhere at all about what specific components in a braking system might have a problem with a change of fluid type. You would think that the internet being what it is, there would be lists of which DOT 3 platforms do not survive if switched to DOT 4/5.1, and why.
I have anecdotal reports from other Trailblazer/Envoy people that they switched and have had no issues, but you know how that goes. Some people think drywall screws and zipties are a fantastic way to do body repairs...
Quick search
It doesn't have to be perfect, it just needs to be done in two weeks...
A wise man once said "google it"
Bill wins the internet today
it really is that simple
dot 3 is for everyday driver that never changes the fluid
dot 4 has higher boiling point (specifically for performance) but does require more frequent fluid changes.
One fluid is not better than the other, they both have benefits and draw backs. They are interchangeable. You will likely never notice a difference with either fluid.
Michael Bray
Rusty Knuckle Garage
Slidell, Louisiana
20yr Master Tech.
Advanced Level Specialist
Custom Car Fabrication, Customization, High Performance.
GM World Class Technician
Shop Owner
Yes, I already found people claiming that. Also found people claiming that there are some DOT 3 systems that cannot use DOT 4, but never explained what those systems were or how you could find out. Hence the thread attempting to explain my uncertainty, which remains.
Wikipedia:
It's that [citation needed] that is the catch. Is the citation missing because that particular claim is untrue? Or true, but simply missing a citation to an outside source?...a vehicle that uses DOT 3 may also use DOT 4 or 5.1 (a temperature upgrade) if the elastomers in the system accept the borate compounds that raise the boiling point,[citation needed]...
I believe your over thinking it
the higher number fluid has a higher boiling point, and therefore needs a higher temperature to "boil off" moisture
lower number is used in vehicles that get less maintenance. The fluid boils sooner, but that also increases its useable life as it can disipate the moisture in the system easier.
Dot 4 has a slightly higher boiling point, and therefore needs more attention as it tends to retain moisture, as the temp required to boil the water off is higher.
5.1 is extreme duty, it has a substantially higher boiling temp, which is high enough it likely wont boil moisture off, and therefore requires frequent fluid changes to maintain that temp.
The boiling point, and the fluids point at which it can disipate moisture are the only differences
Michael Bray
Rusty Knuckle Garage
Slidell, Louisiana
20yr Master Tech.
Advanced Level Specialist
Custom Car Fabrication, Customization, High Performance.
GM World Class Technician
Shop Owner
DOT 3 - glycol ether
DOT 4 - glycol ether/borate ester
DOT 5.1 - glycol ether/borate ester
It seems the implication is that some brake systems may have non-metal parts in them that are not compatible with the borate ester.
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The only way brake fluid gets rid of water is when it gets flushed out and thrown away. No way is there any kind of process where it ever gets hot enough to release any captured moisture while still in the system.
Water still gets into DOT 5 systems, but it accumulates in specific locations and sits there and causes more immediate damage, whereas with the other fluids the moisture is absorbed into the fluid and dispersed and diluted to the point where it doesn't cause corrosion. It has a limited carrying capacity though, it can only hold so much and then needs to be replaced to shed the water.
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