Yes, the proportioning valve on the back of the injection pump (or side of injection pump in the 603 case)is a controlled leak. It might not be good to suck air from the area under the hood (dirt, oil, rain, etc., etc.). They ran the leak line into the dash to pull clean air.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician