Brake pedal doesnt return

70 Cuda with factory power front discs. Car sat along time. Brakes look like new. For the first 10 miles or so the brake pedal won't return on its own. I have to put my foot under and pull up. After a long ride they work fine. Plenty of vacuum. Any thoughts? Booster? Master? Calipers? Weird how it works after a while. Thanks John.

pull the vacuum to the booster.
try again, see if it still does it.
sticky valve in the booster could cause it too.
had one that pulled the pedal down as I drove.

It will be much harder to brake so don't drive too far or too fast to try this.

Did this get resolved? Same issue.