My Silica pumps (I have four) use an oiled leather cup inside the tube. When the pump doesn't have resistance, the cup has curled up just enough to not seal against the tube walls. Usually I can give it a really hard, fast pump, and that opens the cup just enough to touch the tube walls and everything works again. Worst case, I have to unscrew the pump and sort of massage the cup to a more open shape.
Oh, wait - I have the frame pump with Campy head. I'm not sure if that is what you have. Sounds like you have the floor pump.
Anyway, see here for more info
Silca Pump Rebuild 136