No Pedal

restoring my 68 GTS, all new brake lines, wheel cylinders, master cylinder, I bench bled the master, bled all 4 wheels using vacuum pump because I didn't have anyone to pump the pedal, got fluid out of all wheel cylinders, adjusted all brakes, so far so good, now here's my issue I pump the pedal a couple of times everything is fine, release the pedal let stand for 5 seconds or so and the pedal goes to the floor, after only 2 pumps pedal returns to normal, after pumping the pedal I could stand on it and the pedal stays hard, as soon as I release issue
starts again, I checked for leaks, none, is it possible I still have air in the lines?, the only thing I did not replace was the distribution block it's original.