I replaced the vacuum pump on my 124 (603 engine) when it started to leak oil at 110,000 miles. The pump itself looked OK but it seemed foolish to take the chance. On the 124 the pump itself protrudes well into the front of the engine block and resides about an inch from the cam chain pretty much guaranteeing a catastrophic failure if the pump falls apart. On the 123 the pump is actually in a housing that attaches to the front of the block so at least you have a chance that the parts may fall in without taking the rest of the engine with it. Personally I'd replace it if you think it's showing any sign of problem but on the 124 I'd replace it as a regular maintenance item.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold