That sounds right...out of the manifold to the check valve high on the firewall, then down to a T on the drivers side at the Hydrovac, then out of the T and over to the tank on the passenger side. I think 1/2" copper will do the trick!
There could also be a fresh air intake on the Hydrovac that should have an air filter on it or be piped back up to the engine air cleaner to keep dust/dirt out of the Hydrovac piston chamber. Some trucks had what looks like a valve cover breather attached to the inside of the cab either behind the dash on the firewall or behind the seat on the floor.
Your existing MC will work fine...
Mike B
