360 running on 4 cylinders

Was #6 at TDC...... COMPRESSION?
Not that it matters......
cuz, in my experience, from the chamber, cylinder pressure at TDC compression, can only get into the CC three ways, namely
thru the piston,
around it,
or out past a ruptured headgasket..
But that doesn't explain the Vacuum gauge action, cuz the CC ain't connected to the intake, except via the PCV I guess. ..... but that would have corrupted both plenums.
This is getting interesting.