Mike Ryan's note (MMD 980422) reminds me that I obtained many years ago
a device called an 'Airpot'.  It is a air-damping, glass cylinder with
a graphite piston and fixed or adjustable leak valve, one-way or
two-way control.  Very precise damping in the adjustable version from
very stiff to very loose.  I find they are still in business with a web
page at:

    http://www.airpot.com/

Hal Estry