I've been having issues with my brakes since the day I have had the car, but have not been able to figure it out. Basically the brake pedal is hard when the car is off, but goes to the floor when the car is running. The brakes react like they are working properly, but could stop the car better. So far I've rebuilt all calipers, replaced the master cylinder twice, done multiple flushes and bleedings, and replaced the booster. There are no leaks in the lines. Right now I'm thinking it's the yellow check valve, is that a good assumption? What else should I try?

Thanks guys!