I need some brake help please

[nychevyman]

I have a 67 stepside that i have been driving for a few years.
4 wheel drum brakes. worked great no issues.
the pedal went to the floor and stayed there at a stoplight.
I could not pull it up and the spring is fine. i gassed it away from the light and I felt the brakes release and the pedal came up.
with it in my driveway it will sink to the floor, pushing it in just half way it will suck itself down.
if i shut the truck off it will release and come back up in 10 seconds or so.
its a short height, fat booster 8 3/4" around approx
i have 20" of vaccum in the line from the intake
the check valve is good.