Modern Vespa

So. The owner's manual and workshop manual say to do it every 2 years. I've seen many posts here about people doing it.
But, my question is why? It's a sealed system, isn't it? If so, no moisture should get in unless you open it. I've never heard of changing it, nor had it done on any of my cars or trucks in over 45 years of driving.
I've never had a PTW with hydraulic brakes before, so maybe I'm missing something.
I really don't mind doing it, but am just wondering, "What's the point?"

After only a year of service, the brake fluid in the average vehicle may contain as much as two percent water. After 18 months, the level of contamination can be as high as three percent. And after several years of service, it is not unusual to find brake fluid that contains as much as seven to eight percent water.

As the concentration of moisture increases, it causes a sharp drop in the fluid's boiling temperature. Brand new DOT 3 brake fluid must have a dry (no moisture) boiling point of at least 401 degrees F, and a wet (moisture-saturated) boiling point of no less than 284 degrees. Most new DOT 3 fluids exceed these requirements and have a dry boiling point that ranges from 460 degrees up to over 500 degrees.

Only one percent water in the fluid can lower the boiling point of a typical DOT 3 fluid to 369 degrees. Two percent water can push the boiling point down to around 320 degrees, and three percent will take it all the way down to 293 degrees, which is getting dangerously close to the minimum DOT and OEM requirements.

DOT 4 fluid, which has a higher minimum boiling temperature requirement (446 degrees F dry and 311 degrees wet) soaks up moisture at a slower rate but suffers an even sharper drop in boiling temperature as moisture accumulates. Three percent water will lower the boiling point as much as 50%

There's some suggestion that moisture in the system can lead to long term master cylinder or brake calliper corrosion -
http://www.webbikeworld.com/Motorcycle-technical-articles/bleeding-motorcycle-brakes/bleeding-motorcycle-brakes.htm

Agreed, but have you ever seen or heard of this? Just interested.

I have seen nasty calipers, but usually they weren't that young!

I, like Danny, am willing to be re-educated.

The youngest one that I ever saw was on a BMW that was about seven years old: the fluid hadn't been changed in around four years. It was a bike that a friend bought and the front calipers were completely bound up. I helped him take them apart and they were pretty gnarly. The bike may have sat outside a lot before he bought it (I don't really know) but the calipers looked okay on the outside but were beyond rebuilding due to all of the corrosion on the inside. He had to buy a set of used Brembos from a wrecked bike to replace them. It was definitely due to moisture in the brake fluid. I change the fluid in all of my vehicles every two to two and half years. Better safe than sorry.

OK, here's another for you. I have a new, never ridden MP3 which is two years old. Shall I change the fluid?

Pale, straw-coloured, no cloudiness. Does hygorscopic fluid change colour with water absorption?

Agreed, but have you ever seen or heard of this? Just interested.

Steam doesn't compress like a fluid.

The whole point is that gases do compress, where as liquids don't. I too have seen corroded brake lines, but that is usually from the outside. Let's get it straight, I am not disagreeing with anyone in particular, just interested in opinions.

Thanks, everyone, for all the insight. I'm still not totally convinced that it's necessary, but I went and bought a bottle of DOT4 and a couple feet of 1/4" clear tubing today.
I figure it sure won't hurt anything and it doesn't really cost much.
Now I just need to find one of those "round tuits."

Thanks, everyone, for all the insight. I'm still not totally convinced that it's necessary, but I went and bought a bottle of DOT4 and a couple feet of 1/4" clear tubing today.
I figure it sure won't hurt anything and it doesn't really cost much.
Now I just need to find one of those "round tuits."

I'm going t follow cheekythomas advise....
What is the volume of the system, couldn't find it in the manual. Is DOT 5 ok? It is said to be hydrophobic.

The volume is probably a certain quntity - but some is always, always spilt, so what that quantity is is largely irrelevant.

I only change brake fluid if the colour has darkened considerably. This can be in 12 months or five years - depends on usage and conditions.

Just buy a small bottle of DOT4 from any auto parts store. It will be more than enough to do the front and rear brakes.