Problem solved!
Well, thanks to a photo someone else had included in an old post here, I was able to spot the problem, and get the spring seated correctly. The wheel is a bit snugger than it should be, so it doesn't spin very freely, but the darned pedal works again, so I'm happy.
I bought a replacement pedal for my stalwart vac (have done all my own repair work so far), and attempted to install it. Cannot get the spring to provide enough tension for the pedal to work properly. Contacted Simplicity, but all they would say is that their techs used to use a "special jig" to...