A good vacuum shut-off valve will hold the shut-off lever down indefinitely using residual vacuum. You may want to test your shut off valve to see how well it can hold a vacuum, it may be in the process of dying. There is a published specification in the OM603 manual for how much vacuum should be lost over a specified period of time.

It's much easier to change it while the high pressure lines and intake manifold are off for the delivery valve seal change. In reviewing my maintenace records I see that my shut-off valve went bad approximately 10K miles AFTER I changed the delivery seals!!

YMMV.