The brake light is turned on during two events: when the emergency brake is engaged, and/or when there is a loss or reduction in system pressure. It's tripped by a switch that the e-brake contacts when depressed, and is tripped by the sensor in the proportioning valve like you said.
The parking brake switch is under the dash and is contacted by a bracket that's bolted to the parking brake pedal when it's depressed.
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1979 Trans Am W72 400/4-Speed WS6 - Starlight Black Hardtop
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