Having just taken all of my lifters appart, I must agree with most of the
discussions so far.  Except, I don't think they really rely on oil pressure
to "pump" them up.  There is a spring inside the tappets that causes them
to extend to take up any slack if they can.

                              <-- Pushrod end

              * ####### *               Key:  * Barrel
              * ####### *                     # Piston
              * ###                           S Spring
              * ### ### *                     o Valve ball
              * ## o ## *
              * ## S ## *
              * ## #### *
              * S     S *
              * S     S *
              * S     S *
              ***********
                              <--  Cam end

The valve, which is a small ball bearing pushed up by a small spring, allows
oil to enter the bottom of the barrel below the piston.  To get out, the
oil has to squeeze through the clearance between the piston and the barrel.
In other words, the device can extend quickly, but compresses very slowly.
It took a couple of minutes of C-clamp pressure to get mine compressed to
the point where I could fit the circlips.  

OK, now for my question:  I had to take my tappets appart to remove the
air that had gotten into the chamber, making the tappets quite squishy.
How the heck do I keep from getting air in there again.  No matter where
I stop the crank, some of the valves will be open, and the tappets will
slowly compress.  Then when I start the engine, they are bound to suck in
some air.  Do the tappets eventually clear themselves of air by themselves? 

-Albrecht