There should be a check valve in the port of the master cylinder that would hold some pressure in the system when you pump it up. Drum brakes have it, discs do not. If the brakes are not adjusted and that valve is working, you should only have to pump it up once and then the pedal should be "normal". After the brakes sit a while, you would have to pump them up again. Sounds to me like that check valve is not working and the adjustment is off like 3B suggested. I don't know of a way to replace the check valve, I think that would be one for a rebuilder. Maybe someone here knows how to do it.
