Hot water radiant heater problem. air in the system.?

I have a three zone system. I bleed the air out all lines. Start it up when it starts to get cold. As the weeks p, everyday more air get into the line as when the circ. pump starts up I can hear the gurgling of air in the line, especially upstairs. How can a pressurized closed fluid system get air into the line. The water supply line is always on so if I had a leak, which I don't see, the supply line would replace the lost water so the boiler don't burn up, not put air in the line, I'm missing something here. Maybe an expert out there can tell me what is going on? I should not have to bleed air out every few weeks. Thanks, BRAD