Harold , I am concerned about your post .
If the hydraulic system is a closed system , ie , no leaks , and synthetic fluid is being used , then where would water get into it.
My wifes old 180b Mercedes benz has had synthetic fluid in it for over 20 yrs , it sat for 6 yrs at one time , the brakes were still perfect after this .
The statement you make about vapour locking needs qualifying , I think .
Where did you get your data ?
Most of us are aware that regular hydraulic fluid is hygroscopic and needs changing every couple of years for best performance , despite the possibility of and H2O being "boiled off" .
In fact , it is a requirement on my bus to change the fluid every 2 yrs , for its certificate of fitness . Regardless of the mileage covered.
