Dave
This is virtually impossible. If air is getting in, fluid is getting out. When you press the pedal, the pressure in the system may be 2K or 3K psi, but on pedal release, there is not a corresponding high negative pressure to suck air in. That's sort of the basis of the mechanical advantage in a hydraulic system: small force (pedal pressure) on a small piston creates great pressure with little resulting movement. On the return (pedal up), very little pressure is required on the small piston to create great movement (pedal movement).
From what you described, I'd be willing to bet you have an internal seal leaking or a scored piston/bore. Somehow, pressure is leaking internally. Some seals are one-way, meaning there is a right way and a wrong way to install them. Also, it can be quite easy to tear a seal when you're installing it and not know it. Since you are not experiencing external fluid leaks, it has to be an internal pressure leak.
I have also seen people use a small touch of grease on the seal in the cylinder and that seems to make it worse. I usually apply a small amount of brake fluid on the seal before assembly and never had any problems.
Never use grease to lube the seals in a hydraulic clutch or brake system. Always use the native fluid (brake fluid).
Hope we have you pointed in the right direction.
Eric