68 C40 pizza truck Brake issues

[Unofornaio]

HERE ARE THE SYMPTOMS:
1: PRESSING the pedal produces a hissing sound (clearly coming from booster)
2: The vac gauge dropes to zero in concert with the hissing.
3: When pumping the brakes the pedal gets hard and STAYS hard.
4: The initial push of the pedal goes to the floor with the same resistance characteristic of air in the lines, some resistance but pedal is loose and goes to the floor if I push it there.
5: There are NO visable brake fluid leaks.
6: There has been NO LOSS of brake fluid from the resivor.
7: All lines are clear and flowing.

I was going to remove the master bolts and peak at the cylinder just to see if there is fluid but again I have had NO loss of fluid. Meaning the resivor level has not dropped.
One more thing the brake set up and components are all the stock components nothing has been changed or modified.
Single pot master firewall mounted booster and air resivor under driver side behind running board/step.

Thank you