Problem solved. I managed to get a good look at the mechanisms and, with my wife in the seat and pushing the pedal, we jiggled things for awhile. The problem seemed to be a frozen pivot pin (represented by parts 12-15 on the diagram). I used a solvent to clean it out and then lubed it. Works...
I had all that service done in December and ran the mower only a couple of times since then to pull a garden cart. I did run over some gravel the last time I used it, so I'm expecting to find a chunk when I look,
Thanks for the parts diagram. Having that for reference should be a big help. In looking at the diagram, I conclude that the lug on the side of part No. 7 engages the "fin" on top of part No. 13 to keep it from coming backwards when the parking brake is on. Is that correct?