Ok so I just replaced my brake booster because of a leaky rod seal towards the MC. After replacing it, I pumped the brakes a few times and the pedal got stiff. Turned the engine on and it went down to the floor.

driving it around and braking seemed fine and responsive

the issue is when I brake and press the pedal all the way down hard, then lift my feet, it takes the a few seconds before the pedal starts to rise and I can hear a little squeal as if air if is releasing from the booster.

is there a brake in period required? Is this normal for a new booster?

One thing that I will try tomorrow will be taking the MC off of the booster and starting the engine and press the brakes to see if I can isolate the problem...

I didn't see you mention that you bled the brakes. I am guessing you bled the brake lines?

The booster evacuates air from both sides of the diaphragm at rest. When you apply the brakes, the control valve allows atmosphere into the rear chamber to help you apply force. When you release the pedal, the control valve allows atmosphere to flow into the front chamber to release the brakes, then vacuum is applied to both when your foot is off the pedal. It is important that the pedal rod side of the booster allows air in, or the booster will not release.