booster to master cylinder: The rubber o-ring is still pliable but I've never heard of using sealer on an o-ring.

Vacuum pump: Regarding your pump I can only tell you about mine. There is a thin (like paper) gasket between the block and the pump. You can buy it seperately and is also comes with the pump if you buy a new one. You can use a very thin coat of sealer on the block to pump gasket simply to hold it in place during the instal. Dont use RTV.

Between the pump cover and the pump body is a complex shaped o-ring. only use a light coat of oil on this one.

On my 82 300D I have to remove the radiator fan, upper hose, and PS belt to take the pump off (6 or 7 allen screws hold the pump to the block). To remove the pump cover (mine is held down with 6 or 7 slot head screws) I only had to remove the fan. The tricky part with removing/installing the pump cover "on the car" is to get all three pump check valves, their three individual gaskets and the cover's o-ring secured when reinstalling the cover. I used a small dab of grease to hold the cover o-ring in place.

As for your leak I would positively identify the location of the leak by wiping it dry and then looking where the leak propogates from. Then snug up the bolts (don't strip them as they go into aluminum) before contemplating digging in.