First, remove the vacuum hose. Be sure you clean the area around the vacuum unit. You do not want dirt to get into the transmission. Once the dirt is gone, you will see the two bolts that you will need to remove. The bolts hold down the retaining plate. The retaining plate will lift up. With the retaining plate out, you can gently pry the vacuum unit all the way around. Once it is loose, pull it straight out--if necessary, turn it. There is a modulating pressure valve that may or may not come out when you remove the vacuum unit. If it is in the transmission, pull it out. The modulating pressure valve is made of plastic and be sure it not broken.

If you want to replace the vacuum unit, you may go ahead since you have it out. If there is a problem with the unit, your transmission will flare. Also, if you buy a new unit, you will have to adjust it--a trial and error process.

If you want to keep the unit, install the new o-ring on the unit. The measurements of the o-ring from the dealer was 1.34" OD, .932" ID, .102" thick.

To install, place the modulating pressure valve on the vacuum unit and insert it straight into the transmission. Once you get it in the transmission, slip the retaining plate into the unit's slot and bolt it down. Put the vacuum hose on and test drive.

When I did this to my 84 300SD, the unit required an adjustment. The leak affected the shift quality. If your unit was previously not adjusted, I would think your shifts would go back to normal.

It is good you live in Sugar Hill since you can easily visit Mercedes Shop.