Yes. In fact I would recommend that you do so. This has the effect of removing the vacuum reservoir from the system, which I have confirmed has no ill effect at all on the 91....EVERYTHING works perfectly. If you DON'T plug it off, make sure there's no leak in the reservoir. And remember there could be the unintended leak of your disconnecting all the routing on the passenger side....you would need then to plug the other end of the reservoir.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife