Brake Pedal Releasing SLOWLY

Have the Kelsey Hayes calipers on the '70 Duster 340 ... The car has been sitting a long time ... I re-buit the calipers because a couple were frozen .. The brakes work great, except they don't release properly ... The pedal comes up real slow unless I put my foot under it and pull it up ...

What's this mean ?? Bad master cylinder ??

Thanks