FITS THE FOLLOWING: ALL 123 and 126 chassis 240D, 300D, 300TD, 300CD, and 300SD 1980 to 1985 with later style vacuum pump (only one line coming out of top). In 1980 Mercedes changed the design of their diesel vacuum pumps. Instead of a rubber diaphragm pumping action they designed a piston with a 0-ring seal to replace the diaphragm. This design is inherently more reliable but there seems to be a common weakness in the pumps installed on four and five cylinder diesel built from 1980 to 1985 (excluding the 190D).
To determine which type of pump you have you can check the following: Older style pumps have two large vacuum lines attached to the top of the pump and this newer style only has one.
From my experience the most common reason for total loss of vacuum from these later style engine driven pumps is disintegration of the external main check valve. When that happens the pump is not able to build up vacuum pressure and you will have all the symptoms of a complete pump failure i.e.: hard and ineffective brakes, diesel engine won't shut off, hard automatic transmission shifting, door lock inoperable, etc. Before you plan to remove and rebuild the pump I would recommend you first remove and inspect this check valve. It can be easily replaced without removing the pump from the engine. I also have the internal overhaul kit listed if you need one. Look below to order.
This kit includes a brand new OEM check valve and complete PDF installation instructions.
Here you can see the location of the check valve and it is accessible without have to take the vacuum pump apart
How to view the PDF digital instructions:
Once you complete the purchase of this product you will be able to download the digital instructions immediately when you are logged in and on your personal account page (click on My Account). Any PDFs will show up under My PDF Manuals. You will be allowed three downloads, so please save the file on your machine for backup.