Recently my brakes have been acting up. I was going down my driveway a couple of weeks ago and the pedal sank to the floor. And yesterday the pedal was very firm, and today I have to really push on it to get the car to stop. What could this be?

 

Are you loosing any brake fluid if not main brake cylinder is SHOT dont drive it like that.

 

possible split in your brake line. there could also be air in there, and that could be causing the differences in pressure. but these are HUGE mabyes.

first i would check your brake fluid resevoir, make sure the fluid is at max cold before you start it. then after driving for 10 mins or so (i would suggest going on the highway for an exit or so, but ONLY if your really sure your brakes will be fine!), then check the fluid level and make sure its at max hot. if this is an issue, put in some fluid and check after another test run.

ALSO. check for any puddles on your driveway.

if all else fails after this, Go directly to your mechanic and have him check everything out.

ive had this problem before, but when i did my "Brake" idiot light came on.

either way cover all your bases. just think, possible steel brake line upgrade

 

pedal sank to the floor = no fluid in part of the system or air in the line(s).
Start by getting under the car and do a visual inspection of the insides of the wheels. If you see any fluid you probably have a caliper seal failure and you should at the least replace the caliper immediately with a reman. or new, for fronts.
If wheels are ok, check the brake flexible hoses for leaks. Any sign of dampness / bulging / splits / fraying?
Hoses ok? go to the master cylinder. What's the level? Is it changing? Colour of fluid ok? Check the brake pedal inside the car. Is fluid running down the pedal and dripping on the carpet? That's a defective master cylinder.
Another possibility is a brake booster fault, but that's rare and 95% of the time the problem will be something I mentioned above.