"maybe check/replace the vac hose?"

^^
This, I think.

The hose gets soft where the blowby oil is concentrated. That and the heat under the hood, and the flexing as the engine rocks on old motor mounts makes a weak spot right where it goes into the manifold nipple. It collapses under manifold vacuum and you lose power assist.

For now, feel it. Squeeze it. Cut off an inch and see if that fixes things until you can find a fresh replacement hose.
--
Art Benstein near Baltimore

What engineers say and what they mean by it:
"Drawing release is lagging."
Not a single drawing exists.