Sorry, Paul, left out part of it in my haste to finish my post whilst working (looks nervously over shoulder for boss-unit).
IIRC, there was either a spring pushing down or air was blown in from the top to force the balls down the tube, with one or more 'gates' to stop the lower balls from being crushed. I didn't bother remembering exactly how it worked - especially the 'gates' - because I'm not gonna have to fix one!
The long tube was chosen because it guaranteed a smooth, consistent flow of paint, with the air/spring pressure providing the required feed rate.
Now, back to work...