Maybe I’ll say the obvious now, don’t scold me for it... maybe someone will benefit.
And so: brake fluid (any, both DOT3 and DOT4) is very hygroscopic. Only the devil knows where she gets her water from in a sealed system, but the liquid gets it. After 3 - 5 years of operation, the fluid is already quite “wet” and when heated from active braking, the water boils faster than the brake fluid itself. Thus, bubbles of water vapor form in the brake line and braking efficiency drops very significantly, even to the point of no brakes at all. A very unpleasant surprise, fraught with injury, death or prison, and a huge waste of money to solve these problems.
Therefore, I highly recommend buying the best brake fluid (even it will cost the price of a bottle of beer) and changing it entirely, mercilessly pouring out all the used fluid without a trace. In the future, do this regularly every 3 years on your car, immediately when purchasing a used car.
To illustrate, two of our trucks have already lost their brakes this year after going downhill for a long time, in both cases there was some blood and torn metal.

 

That's a fantastic explanation! Thank you so much for that, I've definitely boiled the fluid on some Pennsylvania back roads just following people who ride their brakes. They have tools that test the water content in brake fluid right?

 

After 3 years you know there’s water in the brake system, so just flush out the old and pour in a bleed out the new, or every 5 years and change the Dexcool at that same time.

 

My 2009 came to me with no brake fluid cap. The one I bought from eBay says DOT 3 on it. But have no idea what the original fluid specification was for this car.
What do I specify when I have the fluid changed?

I understand the difference. My British bikes specify DOT 4 because that agrees with the seals in their master cylinders. I don't know why Chevy specified DOT 4 on certain models.
Perhaps only used on SS models? My owner's manual does not specify.

 

DOT 3 OK to mix with DOT 4. Free digital Owner Manual link in my sig. https://www.chevrolet.com/support/ve...manuals-guides
The difference is the boiling point; larger trucks heat the brakes up more. The reason for air brakes on very large trucks.

 

DOT 3, GM P/N 19353126 is the recommended fluid for the HHR

 

With apologies for anything being restated, here’s how I get my head around it:

You can’t use DOT 5 because it’s silicone based and not compatible. It will ruin the system.

DOT 4 and 5.1 have incrementally higher boiling points, higher cost, and increased tendency to absorb water, which lowers the boiling point. They are all compatible.

Spirited drivers will definitely benefit from upgrading, especially if they’ve ever experienced brake fade. In another GM car, I recently had the DOT 3 flushed and replaced. I regret not upgrading, because soon thereafter, I had an emergency stop situation from 70 and the fade was substantial and could’ve caused a crash. I doubt if water absorption is a major issue for most people, but anyone worried about it should just flush the fluid every five years or so.

 

is this statement contradicting itself?

 

DOT 5 and DOT 5.1 are different. DOT 5 is silicone based DOT 5.1 is glycol based. DOT 5 is hydrophobic (does NOT absorb water) the others are hydroscopic (absorbs water).
Some sources say they can be mixed some say never some say yes but with restrictions ( you can top 4 off with 5.1, or 3 with 4 but not the other way around; you can go up but not down; and 5 does not mix at all).
https://www.valvolineglobal.com/en/b...e-brake-fluid/

 

No.

4 has a higher boiling point than 3, and 5.1, higher still.

4 has a greater affinity for water than 3, and 5.1, higher still. As water is absorbed, the boiling point of all three are reduced, because, perhaps oversimplifying it, the boiling point of water is lower than the dry fluids.

I have no idea which is best for neglecting changing the fluid for 20 years. Irrelevant to me.