how is the check valve coming off the booster? when it blows out, will give a “mushy” pedal feel. seeing your car is turbo, it will definitely make this more pronounced. pull the valve off the line from the booster, and blow into it both ways. one way it should blow through, other end it should not.
also, how did you bleed the system? you want to start furthest away from the master. so right rear, left rear, right front, left front.
brake booster just gives aid to brakes. it uses engine vacuum. if a booster goes, pedal should feel like manual bakes–hard pedal feel. a bad master itself will give off a mushy feel.
-Check valve is fine and right direction.
When you had the calipers off, did you check the condition of the sliding pins on both front and rears? The rears typically seize first, as does the screw-in type piston in the rear caliper. If it does in fact sieze, the piston can’t return into it’s bore so it feels mushy even though there isn’t a leak.
I would still go back and re-bleed, and check the direction on the check valve…the arrow faces the IM IIRC
i didnt check the sliders (i’ll do that tomorrow) although i did install a new front right caliper i forgot to mention too.