2001 Oldsmobile Alero brakes

That should probably be fixed before you lose your brakes.

A good suspect when a car has been sitting for a while is a rusted steel brake line. Most commonly the puddle will be under the middle of the left side of the car. Sometimes rear wheel cylinders will leak due to changes in barometric pressure. There are "residual check valves" built into the master cylinder to prevent that, but the brakes need to be operated periodically for those to keep a little pressure in the system. If that is the cause of the leak, you'll see brake fluid running down the inside of one of the rear tires. Simply driving the car will make that type of leak stop.

Rather than covering all the possible causes of leaks, it would be faster if you told me where you see brake fluid or if the master cylinder was just empty. Also, do you have drum or disc brakes on the rear?

Dandy. GM was famous for building the parking brake into the rear calipers, but even with that extra shaft poking out, it is very rare for a caliper to leak. It sounds like you have a simple case of a rusted brake line in the rear. Start by replacing that, then fill the reservoir with fresh, clean brake fluid from a sealed container. Brake fluid loves to suck humidity from the air. That moisture will lower the fluid's boiling point leading to one form of brake fade later.

Leave the last connection on the new line loose, and leave the cover on the master cylinder loose so no vacuum will build up in as the fluid bleeds down. Let it gravity-bleed for ten to fifteen minutes or until you see fluid dripping out. If you get fluid dripping, tighten the line's connection and the cover on the master cylinder.

If you've already pushed the brake pedal over halfway to the floor, we're going to have another issue to address, and you won't get any brake fluid to that line you replaced. You can actually drive the car like that, but lets concentrate on doing it right. If you haven't pushed the pedal over halfway yet, be careful to never do that. Doing so will trip a valve that is real hard to reset until I tell you the secret.

Most importantly, be extremely careful when adding brake fluid that you don't get any petroleum product contamination in there with it. That will lead to a REAL expensive repair that will likely cost more than the car is worth. That includes engine oil, transmission fluid, axle grease, and power steering fluid. Most professionals even wash their hands first to avoid getting fingerprint grease in the brake fluid. Watch out for penetrating oil too. A lot of people run into trouble after using that to get rusted brake lines free.