Slow Returning Brake Pedal Driving me nuts.

Go to each wheel and have some one step on pedal and one by one open that particular bleeder screw. If the pedal releases faster then you know what corner it is an issue.
Do only one wheel per pedal press. Checking fluid level along the way so you don’t run out of brake fluid.
If that does not show a faster pedal then you know it’s in the proportion valve or the master / booster. Do the same with the master two lines (one at a time) to verify it is not the issue. Cracking lines one at a time after the pedal is pressed will give alot of info.