First of all, my 1975 520 should have vacuum controlled headlights. Yes? No? Since my car has not run for years they are in the up position. How do I access this area to check the connections? Any help is much appreciated!
Yes, the headlights are raised and lowered via vacuum. A spring raises the headlights in the event of a loss of vacuum, providing a failsafe mode.
There's a vacuum fitting screwed into the front side of the forward runner of the intake manifold. A rubber hose goes from that fitting to a vacuum reservoir tank in the right-front foot well. The hose runs under the carbs, and diagonally back, right and down to a rubber grommetted hole through the right engine bay wall. Somewhere shortly before passing through the grommet, you'll find an inline brass one-way valve (a Ford part).
Inside the right footwell, remove the outer kick panel to reveal the vacuum reservoir. Near the top is another vacuum fitting with the far end of the rubber hose fitted to it.
Midway between the fitting on the tank, and the grommetted hole through the wall, there is a T-fitting. Another hose runs from that fitting forward over the right-front wheel well. There's a gap between the inner wheel well and the outer fender, and a number of wires, hoses, etc run forward through it.
The forward end of the gap/ passageway opens into the cavity in front of the wheel well, and below the right headlight pod. The vacuum actuator is bolted to the floor of that cavity. An electrically actuated vacuum solenoid switch is bolted to the rear wall of that cavity, which is also the forward wall of the wheel well. A similar cavity and vacuum actuator are under the left headlight pod. The hose from the reservoir 'T's and goes to each vacuum actuator
Early Elites & Eclats had the one vacuum solenoid switch in the right cavity, and it T's into the hose from the reservoir.
Later Elites & Eclats had two vacuum solenoid switches, one similarly located in the cavity below each headlight pod. The two solenoids were supposed to vent the vacuum faster than one, resulting in faster opening headlights.
The solenoid switch is normally closed, sealing the vacuum system. The engine draws a vacuum in the intake manifold, reservoir, hoses and actuators, holding the headlights down. The solenoid valve opens the system to the atmosphere, killing the vacuum, and the spring lifts the headlight pod.
A tight system will normally hold the headlights down for a few days before they start to creep up. Headlights that come up within a few minutes or hours have a leak somewhere in the system. Given the age of the car, the hoses are probably dried out, cracked and leaking. Or the connections to the various fittings are leaking. Those are the easy things. After that, the solenoid valves internal rubber seats (plunger pushes a steel ball into a rubber ring seat) may be dried out or the steel ball may be rusty/ crusty. Finally, the vacuum actuator's rubber diaphragm may have a pinhole leak, or it's steel vessel may have rusted through and be leaking.
Similar but different, a second vacuum fitting is in the back side of the intake manifold's rear runner. A hose from there runs to a T-fitting near the power brake servo (if fitted), and then foward to the cavity below the front-left headlight pod. There it does a U-turn, enters the gap/ passageway over the wheel well, and goes back to a second vacuum reservoir in the left-front footwell.
That reservoir provides vacuum to the power brake servo, and to the vacuum actuators that work the damper doors in the heater/ AC plenum box.
With Zenith-Stromberg carbs, each vacuum fitting on the manifold is fed via two engine cylinders. The vacuum supply is adequate to supply the two critical systems, the power brakes and the headlight pods. With Dellorto carbs, the 4 runners are all separate, and only one cylinder feeds vacuum to each fitting and hose. The resulting vacuum is less plentiful, resulting in noticeable symptoms.
At high speeds, aka more open throttles, the vacuum will be inadequate to hold the headlights down, and they will slowly start to rise as you dip deeper into the throttle.
The power brake servo may start to click in time with the engine, especially if the one-way valve at it's inlet is leaking a bit. The diaphragm will relax between vacuum pulses, the draw down tight again with the next pulse. The root cause is a leak somewhere in the system, including a pinhole leak in the servo's rubber diaphragm. A leak would be a with either Z-S or Dellorto carbs, but the clicking servo makes in more noticeable with Dellortos.
It would help to drill the two remaining manifold runners and install more vacuum fittings. To prevent cross-talking between the runners, each vacuum tap would require it's own one-way valve. Pair two runners to the headlights, and two to the brake servo and ventilation controls.
Sorry if this all too late to be helpful, but I don't often check in here.