Mushy brakes after sitting a few days

[Jim Wright]

Before the holidays in December, I noticed that my brakes ('91 SE-R)  
were just a bit mushy, the pedal seemed to go further to the floor  
than usual, but nothing serious.  I return from vacation after the car  
had sat for 10 days, and when I started it up the pedal went straight  
to the floor and the brakes barely grabbed.

I thought I had a leak, but wasn't able to find anything.  The car has  
basically been a city driver the last few years, no highway time, no  
heavy braking, etc.  I haven't bled the system since I put new pads on  
about 2-3 years ago, could it just be air in the lines, or something  
else?

One thing I did notice is that with the key out, the pedal was rock  
hard, but once I put the key in and turned it, I had mush again.