There is a tab at the top of the page called "New Parts Catalog", which also supports this forum. To keep from chasing the problem, resolve the vacuum leaks first. I found the easiest route to take was replacing all of the rubber flex fittings first then going from there. (This will include the modulator cap.) The modulator replacement should only be done as the last resort as it is a major PIA. Been there.
Try this for the modulator cap:
1984 Mercedes-Benz 300TD Wagon - Automatic Transmission - Page 3