On the way home tonight I started to notice the brake pedal going soft.  I made it to my parent's place, popped the bonnet and saw that the largest portion of the brake fluid reservoir was totally empty.  Odd, because I'd only sighted it full less than 50kms ago.  None of the wheels or hubs have any fluid marks on them.  My mechanic mate suggested that a seal or something has gone, and the fluid has gone into the vacuum servo.  Does anyone have any other ideas as to what I should look for when I get into investigating the problem?
I did think that one day I would change the old booster for one of those biased pedal box setups I've seen - would the cost of getting something like that be much more than replacing the standard servo?
 

sounds like the problem is in the master cylinder not the booster.  The fluid will be coming out the back of the master cylinder and either getting pumped into the booster or leaking down the front of the booster, have a look at it tomorrow, if the fluid is leaking down the front  the paint will be stripped off of there by then
 

Yup - that looks like the issue.  Nice puddle of brake fluid on the road when I checked today.  Seems to be coming out of the booster as well as from the joint between the cylinder and the booster.  Oh well, more work to do!

Typical failure of the master cyl... the rear seal leaks and when the engine is running the booster vacums up the leaking fluid... when engine not running, it leaks down the front of the booster... need to either put a kit through the master or a new master cyl... booster should be OK still.... Alot more expensive to go to a bias box, and depending upon your setup/plans for the car probably not needed either....