The vacuum pump is pretty sturdy on the 82-85 300D cars. However, the check valves can gum up and stick, and there is one check valve on the output of the pump (that is, vacuum side) that is known for breaking or dislodging.

You can take the front cover off the vacuum pump once you pull off the fan. The fan comes off with four bolts. Inside, there are three round check valves about the size of a nickel that you can pop out and clean. On the output pipe, you might find pieces of the output check valve, like a spring and a plastic piece.

On my car, the plastic piece on the output check valve managed to get exactly lodged against one of the other check valves and blocked it off. It is now functioning fine without that output check valve.

Of course, before you tear into the vacuum pump, you really should take a vacuum reading with a gauge, and look for a broken or cracked vacuum line running from the vacuum pump to the brake vacuum assist unit.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles