The hygroscopic nature of hydraulic fluid is there for the same reason it is in car brake fluid-

it's better to evenly distribute water throughout the fluid, than to have it accumulate in a low point

That's actually a really cool way to deal with that problem!

It is. Right up until you go to hit your brakes hard and it all boils out and removes your ability to apply brake pressure in an emergency and you had no idea how much water was in your line because there's no easy way to check, like a low spot with a sight glass.

Or if you leave a barrel of hydraulic fluid outside until it is partially water and it's not easy to notice when you are handling it.

I think my mental model of hydraulic brakes is too simple. Where is there a low pressure region in a brake line? When I stomp on the brakes, isn't the working part of the hydraulic system going to be at higher pressure?

There isn't a low pressure braking region in the brakes.

But brakes can get really hot. Passenger brakes can easily get over the boiling point of water. Keep braking long enough and adding enough heat, and they'll get over the boiling point of brake fluid under pressure.

Thing is, brake fluid is incompressible. Brake fluid vapor however is very much so a compressible gas. Even more so: the water in the brake fluid has an even lower boiling point then that.

So stomping on the braking quickly isn't going to cause much of a problem. But if your riding the brakes down hill for a long distance with 12 year old brake fluid in hot Florida summer on very heavily loaded car? That... might get you into a spot of trouble.

I've gone down the Sonora grade in a Silverado with 22 year old brake fluid. I wasn't towing, but it was very hot out and in general yes the brakes were hot when I checked at around 4000 feet.

I wonder if some YouTuber has done some experiment to see just how bad fluid had to get before a typical driver could notice.

have you considered a career as an F35 pilot? :-)

I take your general point, but as per your specific example, it’s not really possible to go downhill for long distances when the high point of the state is 345 feet.

The one piece of good news is the highest point in Florida is just 345 feet above sea level so I think they'll be okay. :)

Can't agree at all about a sight glass. The lowest point in nearly every brake system is the brake calipers themselves. Wheels going to make it difficult anyways on a car. Not to mention the risk of a rock striking and damaging rigid glass. Or the old interface that's sealing the glass to the calipers wearing out and causing a leak.

It's not worth the expense to manufacture and it's not worth the risk. Not when a litre of DOT4 is $20 and that only needs replacing every 3 years at an aggressive schedule for passenger cars.

Larger vehicles you can purchase a brake fluid tester, but most of the really large ones I know use air brakes anyways.