If you pump the pedal numerous times and then you get a good pedal, then your pads are way out of adjustment. Pumping the pedal will push the wheel cylinder out a bit each time untill the shoe makes contact with the drum, then after you let go of the pedal the springs pull them back again and you have to pump it up again next time you use the brakes.
If you pump the pedal till you get pressure and hold it and the pedal gradually sinks then you have a problem with the master.
If you cant get any sort of pedal then its probably air in the system.
Hope this helps.