I have a 2004 Volkswagen Passat that the brake pedal is really soft (goes to the floor). The driver side caliper piston was replaced (by a shop, not me), idk if that has to do with it or not. There was a leak on the hard brake line going into the caliper. I fixed it and then bled the system (using the two man method). The brake pedal gets hard when the car is off but it goes back to the floor when the car is on. Is there still air in the system? Or am I looking at a MC/brake booster?
It's most likely there is air in the system because the problem started after brake service. You might try doing a full bleed on each corner starting with the one furthest from the master cylinder and working your way to the nearest. If this doesn't work and there are no leaks, the ABS unit probably needs to be bleed. If the brake cylinder ever went dry this is a good possibility. There is a special procedure for this. You can probably find this on the internet. Once you are 100% sure there is no air or leaks in the system and all other brake components are good, then you would start looking at the master cylinder