2020 Superduty F250 6.7 diesel with vacuum brakes. With engine running brake pedal is low at initial application of brakes. Pumping pedal gentle brings it back to normal level, but if you hold the brake pedal on at this moment the pedal slowly sinks back down almost to the floor.
With engine off, after pressing the brake pedal to release the vacuum from the booster the pedal is firm and their is zero abiltity to pump the pedal level up.
Just had truck inspected about 6 weeks ago and the brakes passed the inspection check, Brake fluid level is fine and there is no slow leaks anywhere.
How do I diagnosed this brake issue further without throwing parts at it?

 

I think it's your vacuum booster but not familiar with late model.

 

it could also be a bypassing master cylinder, but i doubt a 2020 would be bad already.

 

I'd guess a bad master as well. Surprising on a 2020. Does it have a lot of miles on it?