[Techno Duck]

Loosing brake fluid is something you should seriously look into. It may be a small leak now and its only a matter of time before it gets worse. Its not to hard to figure out where your leak is coming from considering there are a number of trouble spots which are all fairly easy to check.

I would start by pulling off all four wheels one by one and inspecting the caliper pistons and bleed screws for signs of leakage or excess dirt accumulation. Check the rubber brake lines also. I see your car is an '83, and its possible your brake lines were never replaced. Thats more than 20 year old rubber that has been subject to all of the elements including high pressure. It may be a good idea to replace those anyway. A set of stainless lines can be had for under $75.00 and is a good thing to change regardless if they are leaking or not.

If everything checks out here, inspect the master cylinder around where it connects to the resevoir on top. Also check around your clutch pedal for signs of wetness. Its possible the braided rubber line is leaking (fairly common).

Your spongy / soft (however you want to describe it) brake pedal is a result of the brake fluid leak. Either its forcing fluid out or you have sucked in air through where your leak is. If your slave cylinder was initially leaking, its a good bet your master cylinder will eventually, or has already developed leaks. They see the same amount of usage and should on most cases be replaced at the same time. When you isolate the leak and replace whatever is the problem, you should absolutley flush the entire brake system and get new fluid in there. Brake fluid is clear with a slight brown / gold tint (or just straight up blue) depending on the brand, it should NOT look like CocaCola (what i got when i flushed my system!).