When you ran the climate control unit, did you hit every single button? If there's a leak in one of the vacuum actuators (or the solenoid valve they're connected through) it might only show up on certain settings. And even then it might only happen for a few seconds (I guess the CCU will turn the solenoid back off after enough time to open or close the actuator has passed).

At least, that's how it worked for me when I was troubleshooting the shutdown problems my 300SD was having. It was quite frustrating tracking down the leaks! It turned out three of the solenoid valves behind the climate control unit were leaking like crazy. Replacing them fixed all my problems (but I never had any issues with my brakes).

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)