Brakes

Assuming no air in the system, you might have a bad master cylinder. A pedal going to the floor is not a sign of a bad booster anyhow, but it is for a bad MC.

If you replaced the pads properly air should not have entered the system. Bleed the brakes properly and see if the pedal comes back. You could have a bad master cylinder. The fluid is bleeding back past the piston seal within the cylinder. Also, never push the pedal all the way to the floor when bleeding. Under normal use the piston never travels to its furthermost point. If corrosion builds up at the furthermost point, pushing the rubber seal against it using the pumping action to the floor could damage the piston seal.