When you're driving and have to brake, the feel of the brake pedal should be fairly consistent each time you brake. Obviously, a hard brake for a sudden stop will feel different from a general braking motion as you approach a stoplight. But between similar braking situations, you shouldn't notice a real difference in how the pedal reacts. If you do notice a difference — say, along a street with four stoplights, the brake pedal at the first two and the last felt normal, but the pedal at the third light felt like it sank lower — that's a sign something is up. Normally, something like that indicates that the master brake cylinder might need to be replaced.
Initially the Signs May Be Fleeting
The signs that the master brake cylinder is the source of the spongy pedal are not that obvious. You could be driving along a street and realize that the last time you braked at a light, the brake pedal seemed to go lower than normal before the car stopped, only for the next braking time to feel totally normal. This is a good example of why you want to be in tune with how your car runs because you're the one who can spot weird issues like this. A brake pedal that is occasionally and randomly spongy is not something your mechanic might spot when they drive the car into a service bay for an oil change, for example. Take note, and if you feel anything odd, have your mechanic drive the car for a few miles.
This Is a Repair You Should Have Done Quickly
Anything to do with the brakes should be inspected as soon as you can have it done so that you know what's going on. If you find out that the master brake cylinder is the likely culprit, that's a repair you should have a repair shop do (not a do-it-yourself job), and it's one that you should schedule as soon as you can afford it. While the initial signs that the master brake cylinder needs replacement may be minor and inconsistent, there's no set path for that cylinder problem to progress along. In other words, finding signs now does not mean that you have a specific number of miles before the cylinder becomes a real problem. You have to assume that once you find out the cylinder needs replacement, you need to get it done as soon as possible and drive as little as possible in the meantime.
Because the master brake cylinder controls all four brakes, this is not something where if one part goes, the rest will compensate and let you operate the car normally. If the cylinder fails, the brakes don't work. Have the brake repair done as soon as you can for peace of mind and safe driving.
Contact your auto shop to learn more about brake repair.