A question for car mechanics

[Kansashunter]

My wifes cars brake pedal started going to the floor if you pushed on it like sitting at a stoplight. The brakes worked fine but the pedal would keep going down. I checked it out and it was full of fluid and couldn't find any leaks. I thought it had to be the master cylinder. I replaced it today and the pedal is harder but if I keep pushing it will go to the floor. I did it several times and checked for leaks again and found none. I don't know if there is something on a newer car that I don't know about or I got a bad master cylinder. 2011 chrysler 300. Does anybody have any ideas?

[PFD45]

Anti lock valve???

[Gila]

Check for fluids in the brake booster and for vacuum leaks.

[jbp-ohio]

I had a car that did that. i was down to the vacuum booster or master cylinder so I replaced both at the same time. I think it was the booster.

[Justsomedude]

You could have a bad brake hose. The steel braiding breaks and allows the rubber to expand and contract.

[Japhy]

A way to check vacuum boost vs something else is to test the pedal with the engine off.