First thing I would try is shutting it down with the doors locked to see if it shuts down fast then.... that would/could tell you that one of the locks is leaking. Also check all the connections going through the firewall to make sure they are nice and tight. For working on the locks, if that's the problem, you don't really need a Mityvac; just a piece of hose that will fit over the lines so you can suck on them (sounds crude but it works). No manual is going to tell you what lines do what on the 85..... might show you the main line but that's about it.

If you think it's the locks I'll run you through where to get to the lines to figure out which lock it is if you want. Good luck!

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k