Brake depressed insures trans will be in neutral and the switch will make contact the way the linkage is constructed. Is the actuating spring still in place? broken? missing?
I believe that switch is the only one involved on the 123, so if it IS working, your problem lies elsewhere.
Consult the wire schematic for your tractor (links are here on this site) and do some testing.
Who built your wire harness?
I'd contact them
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