Brake light came on, have not used the car in over six months

First the good news. It is likely brake fluid has leaked out from the rear brakes. You'll find evidence of this by seeing wetness in the inside sidewalls of the rear tires, and possibly small puddles on the ground, just inside the rear tires. Rear wheel cylinders have a tendency to leak, so there's "residual check valves" in the lines going to the rear brakes that keep a small amount of fluid pressure on the rubber lip seals. That keeps the seals pressed against the wheel cylinder housing so they seal better. That residual pressure can bleed down over time, then changes in barometric pressure can let those seals seep a little fluid. It is common for the fluid to leak only long enough to empty the brake fluid reservoir, but not the lines leading from it. That's the good news I'll elaborate on in a minute.

On most cars since the early '90s, there are three switches that turn on the red "Brake" warning light. One is on the parking brake pedal. Another is the "pressure-differential" switch which activates when the two halves of the system don't build up pressure equally, typically due to a leak. The one in question here is the "low brake fluid" warning light. That is the only switch that will turn on just by turning on the ignition switch, before you press the brake pedal.

Here's a link to an article related to this:

https://www.2carpros.com/articles/brake-system-warning-light-on

The place to start is by checking the brake fluid level and adding clean, new brake fluid if necessary. Do not fill the fluid to the top of the reservoir or even to any "full" mark you might see. At the mileage you listed, filling the reservoir half to three quarters full should be sufficient. I'll explain why we never fill it to the top later, once we have this solved.

Once the fluid is around half full in the reservoir, the red warning light should turn off. If it does, we will know for sure the low fluid was the cause. Now we have to address the low brake pedal. Air has gotten into the lines, and since air can be compressed, that is why the brake pedal could be pushed too far. Many people bleed that air out at the wheels, but that takes a long time and a lot of wasted brake fluid. More on this later too.

Since this problem usually leaves the lines full of brake fluid and no air, it is only the air inside the master cylinder that has to be expelled. One way to do that is to slowly press the brake pedal about a quarter way to the floor, wait a few seconds, then let it rapidly spring back up. By pushing it slowly, you'll push new brake fluid into the lines while giving the trapped air time to float back up. Pushing the pedal should take a good 10 to 15 seconds. By letting it pop back up rapidly, some of the air still in the lines will wash back up into the reservoir with the brake fluid that's rushing back. Very often you only have to repeat this a few more times before the pedal feel returns to normal. Any little air bubbles still trapped will wash back while you're driving. That air will not go down toward the wheels. That's why it is usually not necessary to bleed at the wheels. Once you get a good pedal, that residual pressure will have been developed, so everything will be back to normal.

Now for the bad news. Based on the age of the car, there is a very good chance you caused a second problem when you pushed the brake pedal all the way to the floor. There's two "bores", or cylinders, where the pistons travel that are pushed by the brake pedal. Since you normally only push the pedal no more than half way down, those pistons don't travel in the lowest parts of those bores. As a result, it is common for crud and corrosion to develop in those lower halves. The upper halves are the only areas you use, so the rubber seals riding there keep those areas clean. Now, when the pedal gets pushed more than half way to the floor, the rubber seals get dragged over that crud, and that can rip them. This can happen when a driver is surprised by a sudden leak, or when someone uses a helper to "pedal-bleed" the system, and that helper pushes the brake pedal more than half way to the floor. When a car comes in with a brake fluid leak, we often include a new master cylinder, along with the other needed parts, in the repair estimate, so we're covered if we do find a new master cylinder is needed. Sometimes it looks like the old master cylinder will be okay, but very often the internal leakage from ripped seals takes two or three days to show up. It will cause a slowly-sinking brake pedal when you hold steady, moderate pressure on the pedal.

Here's a link to an article that elaborates more on this issue.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

When you add brake fluid to the reservoir, it is critical that it be from a clean, sealed container, and there must be absolutely no hint of any type of petroleum product that gets in with that brake fluid. Even one drop of engine oil, transmission fluid, axle grease, WD-40 or other penetrating oil, or power steering fluid will melt the rubber seals. The only proper fix for that is to flush the steel lines, and replace everything that has a rubber seal or part in it that contacts the brake fluid. That can easily turn into a repair that costs more than the car is worth. Also, brake fluid loves to absorb moisture from the humidity in the air. That's why it's important to keep containers sealed. Moisture in the brake fluid promotes corrosion of metal parts. Since some of that moisture can't be avoided completely, that is why that corrosion builds up inside the master cylinder. Besides that corrosion, brake fluid boils at well over 400 degrees, but the moisture lowers that to near 212 degrees. Very often you won't notice that, but it's not that uncommon for brake parts to become hot, then that heat transfers into the brake fluid which can reach temperatures high enough to boil that water and turn it into a vapor that can be compressed. That leads to one type of brake fade where the brake pedal becomes mushy and goes too far to the floor.

While it does take a lot longer to bleed the system at the wheels, one advantage of doing so is it can remove most of that moisture-laden brake fluid. Every manufacturer recommends doing that at some interval, but very few of us actually do it because these systems cause such little trouble, we tend to forget about them.

Let me know what you find with the brake fluid level.

The standard brake fluid will be labeled "DOT3". That will not freeze in any weather you might find yourself in. For those few cars that require something special, you usually have to search for it or ask for it. Brake fluid is not a petroleum product. That is why the rubber seals are not compatible with oil and power steering fluid.

The reason I mentioned not filling the reservoir all the way to the top is strictly to avoid a huge mess in the future. Your car has the common disc brakes in the front. Those use calipers that squeeze the spinning brake rotor like squeezing a dinner plate with your thumb on one side and fingers on the other side. The typical caliper has a single, rather large piston that pushes on the brake pads to apply them and stop the car. When you press the brake pedal, you're pushing brake fluid down there that pushes on those pistons.

As the linings on the pads wear down over time, the pistons gradually work their way out of the caliper housings. That is how all disc brakes self-adjust for that wear. If they didn't do that, the brake pedal would travel further and further to the floor over a few years as the linings wore down. By sliding out to self-adjust, you always have a nice high, solid pedal.

Where this becomes an issue is as those pistons move out to self-adjust, brake fluid fills in behind them. That lowers the level in the reservoir, but there is enough capacity designed into the reservoir accommodate that. When you see the warning light turn on due to low brake fluid level, either there is a leak that needs to be addressed, or the front brake pads are worn and the entire brake system is due for a normal inspection.

Back as far as the 1980s, it was somewhat standard practice to rebuild the brake calipers with every brake job, but today that isn't always necessary. Instead, we have to forcibly push the pistons back into the caliper housings to make room for the new, thicker pads. Doing so pushes all that brake fluid back up into the reservoir, at which time the level will be near "full" again. If someone had filled the reservoir previously, the excess brake fluid is going to spill over and make a mess. Brake fluid eats paint, so it has to be washed off right away. We want to avoid that mess. That is why at other routine services such as oil changes, we will top off other fluids, but a conscientious mechanic will never add brake fluid.

To address your concerns about safety, if the brake pedal seems to be as high and hard as it normally is, the car should be safe to drive. It's important to know the hydraulic system is split into two parts, so if one half fails, you'll still have one front brake and the rear brake on the opposite side working. There is another problem that can occur that only applies to GM front-wheel-drive cars such as yours. The engineers designed their master cylinder with a valve that trips when equal pressures do not build up in both halves of the system. There is a "pressure-differential" switch that will trip and turn the red warning light on again when that happens. When that valve trips, no brake fluid will flow to two wheels. The advantage of that is if there is a large leak, you won't lose any brake fluid. Part of the problem is this is perfected enough that other than the warning light, you may not notice a problem. There won't be any brake pull to one side as would have happened on older cars, and braking power seems to be normal.

The next problem is there is no easy way for the driver to reset that valve in the master cylinder. Most experienced brake system mechanics know how to do it. If that valve is not reset, the brake pads on one front wheel will wear out much too soon, and those on the other side will look like brand new. Every once in a while someone posts a question here about that after they've replaced the pads for a second or third time. A simple test you can do yourself to verify both parts of the brake system are working is to force a skid on dirt or sand. You only have to be moving at a few miles per hour when you do that. That will show if both front wheels are locking up. If you see a skid mark from just one front tire, tell your mechanic about it and that the valve in the master cylinder needs to be reset. That involves blowing just a little compressed air into the bleeder screw for one of the non-working brakes, then that air is bled back out. The brake fluid rushing back up to the master cylinder will unseat that valve to reset it. This usually takes about 15 to 20 minutes.

The brake pedals on GM vehicles feel different than those on most other brands. Typically, after the engine has been off, the pedal is very hard to push, even if there's air in the system. You'll almost always read here the person observes a nice, solid brake pedal, then, when the engine is started and you get power assist, the pedal falls to the floor very easily.

Another characteristic is when the entire system is working properly, you can still press the brake pedal all the way to the floor. Had you been driving the car and done that, the brakes would have been locked up long before the pedal got that far. This throws a lot of inexperienced mechanics for a loop. They mistakenly think something is wrong when there really isn't.

Yes, as I mentioned, the master cylinder was likely damaged by pressing the brake pedal all the way to the floor. The only solution for that is to replace the master cylinder. With the new, (rebuilt) unit installed, you don't have to worry about pushing the pedal all the way down. It takes a good year or longer for that corrosion to build up that causes that damage.

Here's a link to an article if you care to see what is involved in replacing the master cylinder:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

The good news is this happened as soon as it did. It's very common for the damage to take two or three days to show up. By that time a person may have let their guard down and they are surprised by the sudden failure.

It took 15 years for this problem to develop, so don't expect it happen again very soon.

There are still very experienced mechanics who do not believe this can happen from simply pushing the brake pedal to the floor, but we've been warning about it in our classrooms for decades. You are correct that it can be a natural result that surprises drivers when a sudden leak occurs, and they push the pedal to the floor. That is why I included that we typically include a replacement master cylinder in the repair estimate when a car comes in with this type of problem. Owners might get lucky 50 percent of the time and the master cylinder is okay, but is that a chance you want to take? Especially since it is common for that damage to take two or three days to show up. Some customers want the new master cylinder right away once the situation has been explained. Others prefer to wait and see what happens. The worst case is when no one explains what could happen and they just send the driver on his way, uninformed and unprepared.

We are accustomed to being accused of trying to sell unneeded parts and services, and this is a perfect example of someone thinking that after being warned about their master cylinder, and it ends up not failing. We have to weight our reputation over a potential crash or lawsuit.

I had a brake line rust out on my daily driver a few years ago. I caught it fast enough to avoid running the pedal down too far, but since half of the system was still working, I chose to drive it home over 50 miles, but I had to take it easy. Luckily, that master cylinder survived, but I was worried for a few weeks.

By coincidence, the same thing happened last winter on a different minivan just as I was flying down an off-ramp, up to a stop sign. I was going much faster than necessary, and coming up on a stopped car when I got that dreaded surprise. Somehow I managed to stuff it into a snowbank with less than 15" to a light pole on the right and less than two feet from the car on the left. That could have been disastrous, but luckily it only bruised my ego. Funny thing is, that master cylinder failed three weeks ago, long after that scary incident.