Scott hit the nail on the head with brake fluid absorbing the
water. It then lowers the boil temp and becomes corrosive. 
Your best bet is to flush that fluid every 2 to 3 years. I also
would not use any fluid you have had hanging around the garage for more than a few months if it was opened previously. It killed me to toss a 3/4 full can of premium ATE
fluid for my old Volvo but it wasn't worth the risk. 
Keep in mind that our manual shift Vanagons use the same
fluid for the clutch and brakes. A proper flush will require
purging the slave cylinder too. It would be pointless to
flush one and not the other. You auto guys got it made :)

Bill M 

On Jun 20, 2012, at 7:16 PM, Scott Daniel - Turbovans <scottdaniel@TURBOVANS.COM> wrote:

> On 6/19/2012 7:29 PM, Stuart MacMillan wrote:
>> one on the bottom,
>> where water would tend to collect
> 
> that might be true ..
> but then generally, conventional brake fluid is hydroscopic I think is
> the word..
> it absorbs water.
> It will even turn completely to water eventually, or real water-like.
> I wouldn't expect water to separate and collect in the bottoms of the
> calipers,
> but it can't hurt to bleed and flush there too.