Also if you were taking the vacuum reading off of the vacuum line that goes to the white Valve on the Fuel Injection Pump (the one that controls the vacuum to the transmission) there is a restricted orfice in that line that reduces the vacuum.

So between the low vacuum due to the Check Valve issue and the restricted orfice that would lower the vacuum reading from that line.

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84 300D, 82 Volvo 244Gl Diesel