http://webcache.googleusercontent.com/search?hl=en&rlz=1W1MOOI_en&q=cache:far3kc0DqK4J:http://www.dieselgiant.com/mercedesvacuumtroubleshooting.htm+mercedes+w115+green+and+yellow+vacuum+lines&ct=clnk
This should be a link to Diesel Giant Mercedes Vacuum Troubleshooting Basics. Although I believe it is aimed at the W123 series (climate control, etc. which the W115 does not have) I think it is worth checking out.