Brake pedal loses pressure when the engine starts

When the car is shut off, the brake is firm and gets rigid within a couple pumps of the pedal, but as soon as the engine turns on, the pedal loses all pressure and drops until it's about an inch to half an inch to the floor. Pumping multiple times has no effect and the brake still loses all pressure. I have bled all calipers multiple times, and recently replaced the master cylinder, so I checked the brake booster and figured out that whenever the engine vacuum is being pulled, it causes the brakes to lose all pressure. I did this by removing the vacuum line from the brake booster, and even just me sucking air out of the booster with the little vacuum that my lungs can pull causes the brakes to go soft near immediately. Long story short, no vacuum in brake booster = very firm, any vacuum at all = very soft pedal. Now I know this is normally the booster's job, to make it easier to apply pressure to the brakes, but the brake pedal is going way lower than it should, and losing way too much pressure. The brake system on C3's is known to be terribly designed and it could still be a number of issues, but the fact that this issue only happens once the engine is turned on and the brake booster has a vacuum pulled on it means that either something is wrong with the brake booster itself, or the extra pressure supplied by the booster is somehow finding air bubbles in the system, despite the system having been bled. And this is my main question, for anyone that's had experience with something similar to this, do you think I should go ahead and get a new brake booster, or do you think there is somehow still air in the lines and the extra pressure is causing it to show itself?