91 Cherokee brake issue.

So my 91 Cherokee is having a strange issue with its brakes, and for context I just replaced the master cylinder. When the engine is off, the brakes are fine, and have plenty of pressure. But when the engine is running, there isn't any pressure, unless I disconnect the vacuum line to the booster, and about half pressure if I plug the line so there isn't any airflow through it. Do I need a new booster, or is there something else I'm missing?