First, welcome to OCC.........
In order for the tractor to start, you must do 3 things. (A) Sit in the seat [Seat Switches], (B) the PTO Switch must be in the OFF position, and (C) You must depress the Brake Pedal [Brake Switch]. Have you done all that...??
If you think it is the Brake Switch, it is located at the end of the Brake Switch Actuator Arm as seen in the attached image, item #7...
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Roland Bedell
CC Models: 100, 105, 1450, 782, (2) 784, & 2072
Buy: Made in the USA