Just swap the hoses from the headlamp relays to the opposite actuators. When you try them, if the problem swaps to the opposite side, the relay is your problem. If the problem does not move, the actuator is your problem.

Isn't that easier?

[My bet is on the relay...]

 

That's exactly what I tried but some of the hoses are not really long enough. The results made me suspect that I have a number of small leaks at hose connections yet some don't even have enough extra length to trim the ends. I'm waiting for the new hose set to arrive before I continue.

 

SE Mike,
Just a couple ideas.
I always spray all fasterners with Kroil, it comes in an orange can.
I know when a man makes his mind up to start a job there's no waiting for bolts to soak but trust me soak em go to work and return to it in a day or two and you'll be praising the father.
Everything on these cars is stuck together, spray it or break it off.
From personal experience.

If all your vac is depleting after you shut the car off, your control side
is allowing the diaphram inside the top hat on the relay to drop.
Once this drops the dog bone can be heard dropping and all vac escapes from the vent hole on the bottom of the relay.
Sounds like air escaping from a ballon.
What I have learned form doing surgery to this system for the better part of 2015 is that a slow head light or wiper door is most likely a
leaker at the relay,dog bone inside,or the actuators.
When vac escapes shortly after turning off the car, the control side
drops the relay dog bone and vac escapes. You'll definitely hear it at the relay bottom. It is the entire holding tank empting out through an opening the size of a pencil.

I replaced both my actuators twice. Once with off shore specials which leaked moments after being installed. My problem was I waited till a year later to install so I didnt even think a return was possible.
Then, I purchased a pair of take offs from a later corvette, tested by seller,OEM, then tested by me.
The actuators were pumped up to 15 inches.
The following day at dinner time I checked them. They were down to 5 inches. Both consistent though. This is the best I'm probably going to get out of actuators. They snap up and down smartly and in unison.
Next, I sent my relays to Dave on this forum, he has responded to your thread. He milled the dog bones for a o-ring style seal. They now move the vac smartly from open to closed positions. They are like a controller on the rail road tracks.
They just divert vac where directed.
Under vac the high hat will be holding the dog bone up which should keep the head lights down, and vac will be present on rear actuator nipple in this position.

Moving back it's all rubber, simple its good or it leaks. You are replacing the lines as I did so this should solve any small leaks in the old lines. They get chaffed, pinched and some times melted over 40+ years.

Next is the holding tank, test it again, if it holds move to the check valve.
Test it and then move to the filter.
The filter is plastic and is located in a very hot place for plastic to be.
It is right on top of a hot intake manifold.
It may have a small leak at the seams of the two halves.
They are cheap replace it either way, the inside filtration element is probably long gone.

If you get this far without any failed components, then its time to revisit the control side. Control side has two smaller black lines with a white skunk stripe. One for the HL relay and one for the wiper relay.
They run from the check valve and are always under vac when the cars running.
From the check valve the control line goes to the front port of the HL switch, port closest to the **** side, some older diagrams on the net show it reversed. The HL switch is wide open when off and vac runs through it to the pulldown under the dash. The left is for HL, the right for wipers. The pull down should allow vac to pass and block vac when pulled down.
The wiper solenoid from what I read rarely is the issue but all these components are 40+ years old and I do not realistically expect them to be air tight at this point. But with after market off shore parts just as bad and sometimes worse than what you removed I'd just leave the original part unless it has completely failed.
When the car is right the vac will be the same inches at the manifold and the rear red line at the actuators. If its dropped off there is a leak some where.

That takes you through the system as I did it and I had success.
Report back.
Marshal

 

Just received and installed my Dr. Rebuild hoses and both headlights now fly up and down in nearly perfect unison with the engine idling! I'll wait to replace the control portion of the hoses (I know that part of the system is leak-free) until I'm doing work under the dash.

I checked the relays more carefully and they are OEM and leak-free. Some of the hoses in the car had obviously been trimmed as some were significantly shorter than the replacements. None of the control hoses in the engine compartment had the white stripe so I suspect that they had been replaced.

I do however have a significant leak in the passenger side actuator. Same speed leak from both sides so I'm pretty sure that points to a leak in the main diaphragm. I recall it having a very slow leak shortly upon receipt (the first two were completely DOA) but it has obviously gotten significantly worse in only about 10 months and extremely few operations.

The driver side actuator (replaced at the same time) holds vacuum perfectly.

Any others finding problems with passenger side headlight actuators from the vendors? I think they're all selling the same (imported) product now. In '78 or perhaps a bit earlier they flip-flopped the position of the actuators so on earlier models this is the driver side actuator.

The lesson for others is that even if those vacuum hoses look decent and feel reasonably subtle if they're decades old they are probably going to leak a little at nearly every connection. If your system isn't as snappy as you think it should be or if they don't operate in near perfect unison and you've verified that the other parts of the system are good, suspect the hoses.

 

NOS actuator installed and operation is superb!

Lamp assemblies went up in unison and locked when operated more than an hour after engine shut-down! Had I done more than simply idle the engine I believe it would have had enough vacuum for a full down cycle as well.

This is my 2nd time for vacuum leak search and destroy. Idling and starting approaches perfection.

 

I like happy endings