I have a 1999 Blazer ZR2 and i am going to replace the brake fluid.

I have always thought that DOT 4 brake fluid is better than DOT 3 brake fluid.

I am kind of suprised that the owners manual says to use DOT 3 brake fluid.

Since i am going to replace the brake fluid i am thinking of using DOT 4 brake fluid. I am pretty sure it does not matter if i use DOT 4 or DOT 3

Anyone here on the forum that think this is wrong and that i should use DOT 3 brake fluid?

 

Your question got me curious and then I found this document from Raybestos:
https://www.brakepartsinc.com/dam/jc...ke%20Fluid.pdf

To quote it
"What to Put in Your Car

The simple answer is to put whatever fluid it says on the cap of your master cylinder reservoir. For older models that do not have this information written on the cap, use DOT 3.

In a pinch, remember glycol-based fluids are interchangeable. You can mix DOT 3, DOT 4 and DOT 5.1 and it won’t affect the performance of your vehicle. DO NOT mix silicone-based fluid (DOT 5) with glycol based fluid. They are not compatible".

And then there was this link:
https://www.valvolineglobal.com/en/v...5.1-and-dot-5/

It says
"Brake fluid DOT 3 uses glycol ether as a solvent and a conventional brake fluid used in vehicles produced up until the 1990s. It is suitable for all brake systems and all driving conditions.

Brake fluid DOT 4 uses glycol ether and borate ester compounds which improve its performance compared to DOT 3. Cars produced after 2006 use it as a standard. Like its predecessor, it is suitable for all brake systems and driving conditions."

My interpretation is that you could upgrade to DOT 4 with its higher boiling point and possibly less hydroscopic nature, but it might not be necessary. Do you do excessive braking like from towing when going down large hills? If not, the DOT 3 will likely be fine. It appears that the reason our owners manuals list DOT3 and not DOT4 fluid is because DOT4 did not exist yet.

From the Raybestos source I saw that the DOT3 and DOT4 fluids could look the same so it might be hard to tell when the new fluid has reached your brake cylinders as you bleed them.

To improve my braking, I installed cryo-treated, slotted rotors with heavy duty high-temp pads on the front for when I tow my boat down some long, steep grades and I have had no issues with using DOT 3 fluid.

 

From looking at what you wrote i can use DOT 4 but the best is to use DOT 3 if i got this correct

 

My take is that you could use DOT4 and it will work fine, but it is likely overkill. If the DOT4 is more expensive, and you haven't had any other braking issues, then I wouldn't swap.

As it turns out DOT4 absorbs water more readily than DOT3. That means you have to replace it more often.

"When examining DOT 3 vs. DOT 4 brake fluid, the boiling point and lifespan are the most notable distinctions between types. You’ll have to replace DOT 4 more often than DOT 3 brake fluid." (DOT 3 vs. DOT 4 Brake Fluid: What's the Difference? - AutoZone https://share.google/hYmCQB0TPfJtrsdgS)

This would tip the balance to using DOT3 for me since I've never experienced any issues with brake fluid boiling even with towing my boat down some steep and long grades and I'd rather not replace my brake fluid more frequently than before.

 

I've been using DOT4+ in my 2000 Jimmy for several years, because it provides better performance - e.g. much higher boiling point - than the old DOT3. It's better stuff. It's more expensive, but much better than the factory fill. Regular DOT4 is still better than DOT3, and the price difference between that and DOT3 is insignificant these days, but going cheap on fluids is false economy in my book.