Vacuum question - S Type

[ChrisDixon]

According to the service manual the reservac tank in the heater contol system is fed by vacuum from the inlet manifold, and as I can only find one takeoff which feeds the brake servo, and a pipe that goes from the reservac tank in the left inner wing to the other side of the engine compartment, where the servo lives, will I find a t-piece somewhere in the vicinity of the servo? As I don't recall the heating system having much of a reserve, the air flap tending to slowly close when the engine stops, there is likely a leak somewhere, but my attention is drawn to the check valve, would this prevent the reservac from holding a reserve supply, or is its function to stop it getting too much?