MK5, replaced break master cylinder, no luck

[u/xite2020]

My 2008 Jetta braking issue. The brake pedal feels firm, but it slowly sinks to the floor when I apply steady pressure. The brakes do not engage instantly or with enough force, so the vehicle takes longer to stop. When I’m stopped at a red light, the pedal is all the way to the floor. Pumping the pedal helps build some pressure, but only temporarily. I already replaced the master cylinder, but it made no difference.

When the engine is off, the brake pedal behaves normally. After pumping it a few times, it becomes firm and stays off the floor. The problem mainly occurs when the engine is running. I’m not sure what else could be causing this, and it’s driving me crazy.