Here's a wild guess, a very small vac leak, God knows where (one of the reservoirs?). But it takes time to catch up with the leak, then you don't notice it because it's not there after driving.

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83 SD

84 CD

That makes sense. If so, I wonder: is it just coincidental that the problem developed when I changed the pressure converters? Or did changing the pressure converters cause the leak to develop? OR did the previous pressure converters somehow mask the leak?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife