Upon further inspection, the original intake cap wasn't a filter but a spring loaded valve.  It closed when the tube wasn't connected.  When the tube was connected, it pressed in a spring loaded plunger to open the valve.  A small groove allowed fluid to flow out of the valve.

A lower o-ring was the one that cut off the fluid flow.  An upper o-ring was supposed to form a seal against the tube.  It seemed to be another source of easy air leaks that could require lubrication.  The lower o-ring was not user serviceable.

Your best bet is to scrap the intake cap but keep the spring.  O-rings tend to die in storage, which makes lions wonder how long the pump is going to last.