[Tricky]

As a newbie myself my i suggest you look under the W140 forums for advice!
It seems this is a common fault on these models (mines a 600 SEL), and mine does the same.
Take out number 9 fuse from the fuse box in the boot, count to ten the replace. Apparently this resets the vacuum pump. Worked for me, although i may have to do this several times a week. Of this persists it means the vacuum pump is on its way out, big bucks for a new one.