Brakes locking up

Assuming this was not happening prior, I suspect you have a faulty master cylinder.

More than likely what is happening is the master is getting stuck and when you remove the vacuum booster it is releasing the pedal.

You may get the same result if you pick up on the pedal.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

However, I can't get a good picture of the pedal assembly because it may not pull the pedal back. It may rely on master cylinder negative pressure which means the master is what returns the pedal. However, if it is getting stuck then it will not do this.

Basically, if this just started after replacing the master cylinder, I would suggest replacing it again as the parts store should exchange it for you.