Definitely need a good seal from that square O-ring between MC & booster. Not sure how "spray to check for leaks" was done. There are ultrasonic microphones one can move around to try to "hear" a leak. If not from that joint, the booster itself may be leaking (big rubber diaphragm inside). Did they "tee" in a vac gage to measure what vacuum you have with the booster connected?

I don't know what the "vacuum assist unit" is. My 300D's don't have that. Mine do have the "blue saucer" (vac amplifier), but that is for the transmission modulator signal.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans