617: Probability of fuel shutoff valve failure?

[phasmatisnox]

I bought my '84 300D Turbo with some problems. One is that it does not shut off with the key. I know it has a weak vacuum system, so my first suspect is the fuel shutoff valve actuator on the side of the IP, as seen on DieselGiant. I'd link, but the site isn't responding right now. I traced its vacuum line back through the firewall. What's the best way to isolate the problem? Can I just try to shut off the car by hooking up a hand vacuum pump to it? How can I find out if my vacuum is so leaky that it won't actuate this valve? Thanks!