I replaced the front brake pads on my Civic, but now the pedal is very loose and it takes me pushing it all the way to the floor to stop the car. Do I need to bleed the lines?

Did you open a line or bleeder port during your brake job. Can you pump the pedal several times in succession to get a stiff pedal, does it get soft again later? When you pushed the piston back into the caliper, did the seal leak. When you push on the brake pedal now, is the piston on both sides working, maybe one is stuck. Answer these and the answer will follow.

Hopefully, no damage was done to the braking system, before removing the cap to the master cylinder reservoir. If you don't remove the cap, first, it's possible to damage the ABS or seals in the brake system.

Did you open a line or bleeder port during your brake job. Can you pump the pedal several times in succession to get a stiff pedal, does it get soft again later? When you pushed the piston back into the caliper, did the seal leak. When you push on the brake pedal now, is the piston on both sides working, maybe one is stuck. Answer these and the answer will follow.

I didn't open any line, but there was some fluid leaking when pushing the piston back in. When I pump the pedal, it gets stiff, but then after starting the car it is soft again later. The piston on both sides appears to be working.

I believe that air got in through the leaking piston seal, the leak you saw made its way through two seals, the REAL piston seal and then the dust seal THEN you saw a leak, bleed the brakes and it should fix it up but BEWARE, brakes are nothing to play with, if you don't feel comfortable change the calipers and rubber hoses for safety. Most of all for the moment, be prepared for total brake failure so do your stopping early and don't wait till you are too close to rearend someone.

Thanks for your help guys. I did bleed them and everything has been working like a charm.