I just bought my first MB. It is a 1988 300E with 124K on it. Had mechanic look it over and other than small things in great shape. The front door locks work but the back door ones move down a little but that's it. I know they work on a vacum and I can hear the pump running either under back seat or in trunk. The gas door locks as well but not the trunk lid. From the manual all the locks should lock when you use the front door key locks. Can anyone give me an idea the best way to trouble shoot this problem and get it fixed.

Thanks for your help in advance.

88 300E
95 Volvo 850 Turbo 115K