GMC Repair: El Camino brake question..., decent shape, master cylinder

Question
Jim:
It's the damndest thing...
I've got a 1979 El Camino (350 engine) in pretty decent shape.  It sat for several years, and...while I know that's never good for a vehicle...it only has 75,000 miles.
Lately, when I add brake fluid and pop the little plastic top back on the master cylinder, a small leak appears right where the master cylinder is attached to the booster cylinder.  A small bead of brake fluid begins slowly trickling down the booster cylinder, sometimes even dripping onto the ground.
The drip doesn't start until I pop the little plastic top back on the master cylinder, so I guess there has to be a small amount of pressure on the fluid.  Also, the fluid level in the master cylinder never drains completely out, but stops about halfway below the "Full" line on the master cylinder's side.
Is there a gasket between the master cylinder and the booster cylinder that could be worn out or defective, allowing this small leak?
Any suggestions, advice greatly appreciated.
Warm regards,
David Gardner
davidg32@yahoo.com

Answer
There is a seal on the back of the master cylinder, however I don't know if you can get it without getting a whle new master cylinder. Also when you remove the master cylinder check for fluid inside the booster by puting your finger in it, if there is fluid in the booster it should also be replaced.