Classic/Antique Car Repair: El Camino brake question..., decent shape, fluid leak

Question
Mr. Benjamin:
This is not exactly up your alley, but thought it couldn't hurt to ask.  Hope you don't mind:

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
Yes, there is a seal around the main brake actuating piston that will allow fluid to leak back into the junction between the booster and the master cylinder.  This is a cup seal, and if it is leaking, that pretty well tells you that your master cylinder is due for attention.  You need to either replace it with a new one (which I recommend because they are dirt cheap now) or rebuild it with a kit.  The kits cost almost as much as a new unit, though, and you'd still have to hone it to get the crud and any minor surface defects off the bore.

There should not be pressure in the reservoir, as the cap should be vented.  I suspect either your vent is plugged with crud, or the beginning of the fluid leak is related to your having just filled the reservoir, rather than you putting the cap back on.

As you no doubt know, the fluid is highly damaging to paint, so if it is running down the firewall, it has already done some damage there - wash it off with plain water, and if the paint has peeled off to bare metal, touch it up or at least wax it!

Replacing a master cylinder is very easy, but you do need to know how to "bench" bleed the unit before installing it, and you need the right kind of line wrenches for the fittings.   The bleeding fittings and tubes will come with the new unit, if you buy it from NAPA - usually the best quality in the aftermarket business.

Dick