I had a similar problem with my 300sd a few years ago, the cars history showed a replacement 10 yrs prior so I spent $20.00 and been fine.

While under the grill for the mono valve be sue to check them little hoses chances are they are original to.

Also as side note I replaced all the vacuum pods for the climate control main flap etc,etc makes a world of difference having the air come out we're it's supposed too.

John