'87 300TD won't shut down
 

I'm guessing this isn't a rare problem, because I encountered this in the descriptions of several I saw while hunting for mine. The seller didn't tell me about it beforehand, so it may be a new problem since he last used the car more than a year ago.
When I turn the key off, the shutoff lever on the side of the pump will go "Almost" all the way down, but not enough to kill the engine. With my hand vacuum pump, I confirmed that the vacuum solenoid is working. The source of vacuum seems to be off a "T" piece in the line to the brake booster. The line goes into the back of the dash, then back out to the injection pump. Is there a valve which opens on the key tumbler, or some seperate solenoid valve? If I re-route the line from the "T" piece, to another source of vacuum, the engine shuts down promptly when the key is turned off. This "T" piece looks like it might be a restrictor valve, but it seems not to be supplying enough vacuum. Any help here would be appreciated.
Also, does anyone have a reccomendation for a good Hard copy service manual?