Changed front rotors, pedal still goes to the floor

There won't be any air in the lines if the hydraulic system wasn't opened or parts in it replaced. By far the biggest cause of loss of pedal is the brake pedal got pushed all the way to the floor.

To make room for the new, thicker rotors, the pistons had to be pressed back into the caliper housings. Once everything was reassembled, you have to press the brake pedal multiple times to run the pistons back out to adjust them. At first you will be able to push the pedal all the way to the floor. That must be avoided unless the master cylinder is less than about a year old. Brake fluid sucks moisture out of the air. That leads to corrosion inside the metal parts. Crud also collects in the lower halves of the bores in the master cylinder where the pistons don't normally travel. When the pedal gets pushed over half way to the floor, the rubber lip seals are run over that crud, and that often rips them. The symptom will be no pedal pressure, or a slowly-sinking pedal. The slowly-sinking pedal commonly doesn't show up until two or three days later.

To avoid the possibility of causing this damage in the master cylinder, professionals never allow the pedal to be pushed over half way to the floor. You may get more ideas from this article too:

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

If you do need to replace the master cylinder, take a look at this article first:

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

Here's a trick that makes this job a lot easier. Loosen the two line nuts just a little, then unbolt the master cylinder from the power booster. Pay very close attention to the steel lines as you turn the nuts to be sure they're not twisting. If the nuts don't spin freely, the lines will twist and snap off. Then you have another repair to do that involves new fittings and making double flares. Double flares are hard enough for a professional. They're harder to do in the confines of the engine compartment. Pull the master cylinder forward off the mounting bolts, then use it as a handle to twist the two lines upward just enough so those first two inches are not parallel to the ground. Then you can remove the lines. This will prevent the brake fluid from running out of them. Be careful to not drip brake fluid on the car's paint.

After bench bleeding the new master cylinder, leave the reservoir at least half full of fluid. When you install it, you will have to tilt it to line up the ports with the line nuts. Remove one plastic bench-bleeding fitting, then install the steel line nut hand tight. While you do this, fluid will be dripping out, keeping the line and port full. If the nut doesn't thread in easily by hand, it's cross-threaded. Start over and try again until the nut goes in by hand, typically four or five revolutions. If you damage the threads in the aluminum ports, there will be no warranty and you will have to buy another master cylinder.

Once the first line is connected hand tight, do the same thing with the second line. When both fittings are hand tight, twist the master cylinder and lines back down to their normal position, and bolt it to the booster. You'll need a helper for the next step. There may be a little air in the ports and tops of the lines. Tighten the front nut, then loosen the rear nut about a quarter turn, then have your helper push the brake pedal down very slowly. It should take him about 15 seconds to push it half way to the floor. Any faster, and the air might still get forced down the lines. While the pedal is moving down, you will see air bubbles coming out around the nut's threads. When the bubbles stop appearing, tighten the nut. Tell your helper to not allow the pedal to move back up until you tell him the nut is tight. Raising the pedal too soon will allow air to be drawn back in through the nut's threads.

When the nut is tight, tell your helper to allow the pedal to come back up quickly. The brake fluid rushing back to the reservoir will wash any air bubbles back too. Do this procedure a second or third time until you don't see any air bubbles, then do the same thing to the front brake line.

As long as the brake pedal is pushed down very slowly, no air will be pushed down the steel lines. The fluid will go down, and the air bubbles will float back up. When the pedal is released quickly, any air bubbles that stick to the lines will be drawn back up to the reservoir along with the returning brake fluid.

Fill the reservoir with clean brake fluid from a sealed container to the same level as was found in the old master cylinder. If the level was fairly low, that's what happens when the front brake pads are worn close to the end of their life. Installing new pads results in the fluid returning to the reservoir and the level will go back up to "full". If you fill it before that, the fluid will spill over and make a mess when new pads are installed.

There's one more thing to be aware of, but it only applies if you were replacing hydraulic parts like calipers or lines. If the reservoir is allowed to run empty, air will have to be bled out of the anti-lock brake's hydraulic controller. On most vehicles a scanner is needed to command the ABS Computer to open two valves so the trapped air can be expelled from those two cambers. Without the scanner, you'll never get a solid brake pedal.