a bad booster can cause an extremely sensative pedal in some instances while still allowing boost, as stated earlier. i've never seen a sensative booster cause a pedal not to return, but it seems possible.
a master cyl can go bad in more than one way, and new does not always mean good. it may also not be matched to the pushrod length. a short pushrod might cause something like this.
the pedal return spring is coiled around the pivot rod, and may be hard to see. check those bushings too, even though the pedal moves freely. do not add an extra spring as a band-aid.
