1990 Suburban dies intermittently

[SoCalSlaughter]

Ive checked all the knock sensor and starter wires, I had the transmission off not to long ago to install a new flex plate and I rewrapped all those wires. I work with a Ford master tech of 35 years, and he says it sounds like its my brake booster. Because the whistle and the dying only happens when I come to a stop, he said it can be a large vacuum leak from the booster. But I dont have a hard pedal, its actually quiet soft. Goes almost to the floor and the truck stops pretty well with having 35" tires on it. He said to try putting the truck in neutral next time im coming to a stop and dont hold the brake and see if it has the whistle or dies. hmmmm