80 300sd slow to shut off( 10-15 sec) Hook mity vac to a "t" in the shut off line(all other vac lines sealed). start engine and vacuum builds to 20-25 inch in about 3 sec, shut off key and vac drops to zero and then builds up to 10 inch at which point the engine stops. have cleaned the lines to the key switch,what else should I check?

Have you tried pulling a vacuum on the line with the engine not running and the key in the position to shut off the engine? I'm guessing one of the rubber connectors on the lines is leaking or perhaps there is a leak in the switch itself.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Yes I used the mity-vac to pull a vacuum with the key in the off and on position, It will hold vacuum in either position. However it still drops to zero when you change the key position.

Then I'd say bad switch. However, I've never had one apart and don't know how and why they fail so don't act solely on my hypothesis. I'd probably get one from a junkyard and switch it out.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13