Brake mystery - Ford F150

Brakes feel like they’re engaged at all times - even when not pressing brake pedal. I replaced calipers, master cylinder, fluid, pads - emergency brake is working correctly as well. Brakes work well to stop the truck but then feels like they don’t let off easy when time to go. Could it be the brake power booster?

Maybe not the brake booster, but instead the brake booster check valve.

Tester