I have a '98 Blazer as a 2nd vehicle with 4 wheel disc brakes. Besides once amonth when I start it up to charge the battery and circulate the oil it has been sitting for about 3 or 4 months. I tried to drive it last week, but when I got in the brake peddle went right to the floor. I pumped them for a while with no luck. There were no fluids on the ground and the master cylinder was full. There were no warning lights on, and I didn't have time to mess with it. A week later the peddle went to the metal again, but this time the brake warning signal was on. The last time I drove it, 3-4 months ago, the brakes worked fine.

Bleed the brakes?

Gees a jimmy for your 2nd car isn't all that. Whats your DD a geo?
If I had the opertunity to get a second car it would either be a cvic for DD or a corvette to store as a second car. I would get a 98 jimmy for a DD tho?

if ur sure there's no external leaks from the calipers or hoses then I would assume an internal leakage in the master cylinder. Meaning the master cylinder probably needs to be replaced, as for the brake warning light that usually indicates low fluid or low pad material. Check those out.

have someone pump the brakes slowly. take off the lid of the master cylinder look and see if theres a little fluid movement, like tiny geysers.
that usually means the fluid is returning back to the master under pressure.
If no pressure, master cylinder is junk.
if the fluid is black, you needed to replace the fluid anyway. now you are going to have to.
pump and bleed the brakes until new fluid comes out the wheel cylinder/caliper lines with the new master cylinder.
bench bleed the master cylinder before you install it, and be sure to keep the reservoir from going empty inducing more air in the lines as you keep refilling the reservoir while you bleed the brakes