Brake Pedal SOMETIMES goes to the floor

[kcackler]

I've got a 76 C10 with a problem. The brakes work perfectly all of the time (They are brand new) and stop me in an instant. However, recently, if I'm sitting at a stop light and sort of let up on the brake to creep forward, when I go to press the brake again, I have no brakes and it goes to the floor. But then all I have to do is let up all the way on the pedal and push it and it's as solid as it always has been.

So what could my problem be? The most I know about brake systems is changing the pads and bleeding the lines.