Shut off valve testing.

I just installed a new shut off valve (in the IP, not on the key switch) on my '84 300D turbo. The car still is not shutting off properly (it slowly winds down then shuts off, takes up to 10 seconds sometimes). In my testing, I pulled a vacuum on the shut off valve, and it takes a few pumps before actually climbing up to the 15-20 inch range, and then holds. I have a feeling it winds down that way because, like pumping it by hand, the vacuum pump on the car needs time to increase the vacuum, and as the engine slows down, it just lengthens the necessary time, does this make sense?

Is my new valve faulty, or is it supposed to take that amount of time to build up? Would anyone be kind enough to pull a vacuum on their known good shut off valve and see if it's the same? Thanks!