[Lynn Eades]

So are you guys implying the stuff just evaporates?
The reality is it has leaked past the cups and out of the boots. if it does it at the master cylinder, the leak should be able to be observed and also at the wheel cylinders it should be able to be seen at the bottom of the backing plates

In the end of the master cylinder (first bits in, rubber seal and a tin bit with a rubber seal in the middle) is a residual line pressure valve (drum brakes). This is there to keep a "column of fluid' in the lines so as to reduce the amount of pedal travel required. This also keeps a little pressure on the cups and keeps the lip of the cups sealed against the cylinder bore. If the vehicle sits for a while this pressure can "leak off" (back to the reservoir) This then lets the cup lips relax back from the bore which in turn can let fluid seep past the cups. (Think about the grooves carved in the bore by even the finest of hone stones- little tracks for fluid to seep from) When you use the vehicle again, a small amount of fluid will probably burn off with use and you may not notice that it had leaked (or the loss of efficiency) Likely not much worse than driving through water and having to dry your linings out.

In your case Phil, I would be surprised if you now find anything wrong.
Knowing that you do your best to keep things in good condition, your system is far more likely to leak at rest (no pedal pressure) than with 35 pounds applied to your pedal. If it is leaking anywhere else a careful look must pick it up.

As an example, if you have ever had a vehicle with a tandem system and if you have ever rested your foot on the pedal and it slowly sunk (internal leak)
Quite often if you push harder, it will hold (not always, and yes, it is time for repairs)