I just found four bottles of Honda Dot 3 brake fluid in a box in my apartment. I'm not going to use them because I don't trust that the fluid is still good.

This site says unopened bottles of brake fluid should be good for 2 to 5 years. Question: Why don't I believe what I read on the Internet. Answer: I don't know.

By the way, over time, the bottles collapse.

 

From what I've gathered over the years, DOT fluid absorbs water from the air. I'd bet that your bottle is still fine, but a new bottle is affordable enough.

 

Agreed. And you are correct. Brake fluid is hygroscopic. Any exposure too air and it will absorb moisture.

 

You can buy a cheap "brake fluid tester pen" from Amazon which will tell you what the water content of your brake fluid is. Note that after using such a pen you have to clean off any brake fluid residue because brake fluid will destroy the plastic on these devices. This is a handy thing to have although not something that you would use a lot.

 

The fluid will be fine if the bottles are sealed. I have never seen a sell by date on brake fluid bottles.