Welcome to 2CarPros.
Any time the pedal goes to the floor, either there is a leak, air in the system, or the master cylinder is bad. If nothing was opened up, there shouldn't be air in it. When you replaced the rear brakes, did you properly adjust them? If they are not adjusted properly, it will give you a spongy pedal, but that wouldn't cause the brake pedal to go to the floor.
Take a look through this link:
https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor
Let me know if this helps.
Take care,
Joe
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Monday, April 12th, 2021 AT 9:58 AM