I was thinking about a relay, but I was thinking of wiring it the way they did in this Hot Rod article. They recommend using a relay for power already. It seems weird to use 2 relays inline. The first would get power from the brake light power and would only send power out of the relay when the brake lights are off. That would power the second relay to power the converter. The article says to use the low current from the relay for the pressure switch and the high current for the TCC solenoid. I may be making this harder than it needs to be. I have new power brake under the floor and a new working brake light switch. They just don't offer the duel switch for this like they do for 80's cars.
https://www.hotrod.com/articles/hot-...hout-computer/