Will DOT 5 fluid absorb moisture like DOT 3?

DOT5 will eventually absorb some moisture, but not like DOT3 will in comparison....

Yes, although DOT5 is often advertised as being non-hygroscopic (not moisture-absorbing) it will absorb a tiny amount of moisture (on the order of 280 parts per million, or .0028%) and then absorb no more.

Absorb? No. That doesn't mean moisture won't get into the system through contamination, humidity, etc.

DOT 3 and 4 will absorb water. Dot 5 will not. Instead it allows it to settle in the system and cause corrosion. Another reason I don't use it in race cars where systems are (or should be) maintained regularly..

No, dot 5 will not absorb moisture. I've had it in my olds for over 40 years. Sometimes the car sits for years un used. I live in an area where it can get very humid in the summer. I have never had an issue with anything in the brake system rusting, seizing, leaking, or any other type of failure. I pulled the car out of 23 years of storage, and the brakes were still fine.

The reason I ask is I took my brake valve apart and the spring and the end of the bottom plug were rusted. It's a repo that was installed sometime in 2014 and always had DOT 5 fluid. Also I have a dehumidifier in the basement where the car stays.

I would say you got a part that was likely not stored properly. I believe it does get quite humid at times in other parts of the world

FWIW, with Dot 5, whatever moisture may be in the system accumulates in little pockets instead of being easily absorbed by the fluid. Maybe it just picked your brake valve as a place to collect.

FWIW the military and the USPS used DOT 5 for many years as their brake fluid and maybe still do (I worked for a brake parts manufacturer until 2005 so haven’t kept track of things like that in the 17 years since I got laid off).