On my 1980 300D, I have a small brake fluid leak. I seems like it is leaking between the booster & MC. This indicates that the MC is probably leaking, but the break pressure is firm & consistent.
Has anyone else experienced this leak ?

You said SMALL... that just means your watching your car , saw the first indications of the leak and have time to prepare your attack on the problem... of course it would be good to clean the wet area and keep checking to see if this was just a spill from filling the vessel... not likely... but just to give you peace of mind that you are going to attack an actual problem...when you do start to do it... if you have a digital camera please take lots of pics of the procedure for the FAQ pages.. Thanks, Greg

The rear seal on the master is leaking by. This usually does not effect the braking, but you will notice that fluid needs topping , but a leak can not be found. The fluid goes into the booster
and is aided by the vac of the booster .
Once the master is fixed, it is wise to put a small hose into the booster through the master mounting hole a suck the fluid out of the booster housing.
I have seen them fill right up... a sign is brake fluid in the booster vac hose.