My guess? In the booster.
But it's only a guess. I'd need to run some tests to try to verify it.... and they might very well end up being inconclusive.
However, the next time you get in the car, try this.
Without the engine running, pump the brake pedal. It should become hard and it's travel should shorten, "topping out" more toward the top.
Then, while pressing on the pedal, start the engine. You should feel the pedal drop and the "feel" become softer.
That is a "quick test" of booster operation. If you don't feel these things, you may want to disconnect and plug the tube going to the check valve on the booster. If that has no effect on the brake pedal, the booster isn't working.
Okay, now for the fun part. IF the check valve hung up and is not "unhung", everything may act normally. That would be the "inconclusive" part of the test.
Post the results.