The prop valve electrical connection is for a warning light that would indicate a single brake circuit failure/leak.
This era of truck doesn’t have an indicator on the dash for that because they came with single circuit braking systems. This era of trucks also doesn’t have an indicator for the parking/emerg brake. If you wanted the warning light, you could adopt the process below to create a dash warning light (I used the tandem lock dash slot for a parking brake indicator, but you could use it for a warning light as well - or do both, there are 2 unused dash slots that are present in light duty trucks that are utilized in medium duty trucks).
http://67-72chevytrucks.com/vboard/s...d.php?t=419932