Let's put it all in a nutshell. First is a complete inspection. Pull all the drums and check for leaky wheel cylinders, like 53 4 me said. I would bet that Ken is right, you have at least one leaker. Replace them all if you do. Put the drums back on and adjust the brakes. If you can, suck all of the fluid out of the master cylinder, you need to get all the gunk out of it. Fill it up from a fresh bottle and bleed each cylinder until clean fluid comes out. Don't forget to keep the m/c full. Once the bleeding is over, push the pedal down as hard as you can and hold it there for a minute or two. Usually takes both feet. IF the pedal suddenly drops, you probably have a bad m/c, if it goes down slowly it could be the m/c, a leaky line or wheel cylinder, or you didn't get all the air out of the system.
