Actually Art, I have looked into repairing these valves. Looked, is the operative word here!

I have two of them in a box under my bench. One, is off my 78 that I replaced from Napa so many years ago that my brain does not want to scare the other half, that still is working, with that date in history!

Just a couple years ago, I snipped another one out of a junkyard car, hoping it might be good enough to put in one of the two cars that needs one. I mean, waiting, until it "really" needs one as in leaking!

Yes, this stuff is up my alley but the alley under that dash, does not have my name on it, yet! I am sort lazy that way. Nothing about being under there, raises the curiosity bar high enough for me, like it does you.

On the feedback statement, you are right that there is no true "bellows" but it does have an expanding "cylinder with sealed in piston" that pushes a lever and fulcrum setup to actuates the valves seal tipped rod.
If the sensing bulb or tube in our case still has its charge of gas (most likely a refrigerant) then an adjustment should bring the mechanism back into range. The mechanism works under quite a bit of spring pressure to allow a trip off point when shut down. I surmise that working the lever promotes a flat spot on the cam and causes the range to fall off. It the turns into a normal on/off valve. As my wife says it either on or its off. It does not help when she want heat like yesterday and wham over it goes either!

The fifty year old technology term you mention is cutting it short several years. Today's thermal expansion valve (TXV or TEV) is still the best "self powered" apparatus going for metering large amounts of refrigerant into a coil. The power head can be replaced without opening the system if it fails.

At first I had planned on tearing up the first one with the bad seal to see if I can make another seal for it as i believe the sensor still worked. I learn what ever I can from something bad, before I throw it away.
The sensing system will be kept if good and interchanges. I could possibly refilled one by making up a "Y" shaped process tube. I' ll pull a vacuum on the tube tosee if it holds first or why it does not. If its good or can be made good I will then fill the tube with a refrigerant. I pinch off the tube to shut it and solder it to seal it.
All this shoud replicate the manufacturing process.

I have been meaning to play with them by setting up a box with a thermometer in it. Getting some experience of how it really works on the bench.
I will use my heat gun or just a light bulb to preset the range of each new or old one before I'll ever try to conquer it down in those tight quarters, with an Allen wrench, that happens to come with running water!

I have also pondered the use of electronic valves you can find in Graingers and I am sure there are other better sources. I think a small servo motor to actuate even a manual valve is very feasible.
A simple controller, possiblly using the "ole time" Wheatstone Bridge circuit. I only know about it because I once repaired the circuit of a variable speed small Hardinge turret lathe before. It was another side job you end up doing for a very small company without dedicated maintenance personnel. My Dad's background caused me to try that one. You know, now that I think about it, it was a broken wire/ solder connection, just like our relays!

That circuit should be child's plays for you. The circuit has been around as long as TXV's. I know that these are all analog ideas. A step motion actuator motor uses a digital input but getting 90 degrees of rotation on a ball valve should be no sweat unless it takes a lot of inch pounds of torque to turn a ball valve of which I doubt.

With "I-Pad" type controllers going to cars, the mechanical thermostats used in refrigerators and ovens bowing down against electronic acceptance its the biggest reason for Ranco to drop making our type valves. There is a lot less tooling and skilled labor involved too.
Those tiny thermistors can be used for "silly" things like GM's individual temperature controlled vent outlets. Or some other sensors to activate a butt thumper, to wake up the brain!
(:) I imagine those promity sensors fit into recliners too! So I'll be awake ever time the wife get close or with a condenser mic, thump me if it hears me snore! (:)

You know, I am very weak in electronics but I know there is a way to translate or a jump around on this simple stuff but car makers never keep it simple. In fact they go out of their way to keep one foot in the air with model changes. All those combination black boxes with a fake expense tag from dealers called "exclusive." New luxury cars, no thanks!

I have been watching the growth of Ardurino, a programmable controller board company. I am sure "garage tweakers" will make more goodies simpler with their help. Radio Shack has even picked up on it and offer boards. They know they have lost customers (electronic tinkerers & those leather people) that made RS/ Tandy in the years past. America needs a different shots in the arm than what their find today. IMHO

My problem, time is not of an essence as much as, it should be.
I am either too cheap or too lazy to get all enthused, as I am not like Dave, the entrepreneur.
He must love what he does but my many loves are turning into occasional "spurts" anymore.

I see Dave's effort just a tiny bit short of something great. His valve is universal enough to work on a farm tractor. It would get better sales with several more makes of cars to use it on if it had the sales pitch of " A retrofitting, electronic, self regulating heater control valve" for your older Chevrolet's, Ford's and imports vehicles. J.C Whitney bound that "KIT" could be?

But I am just another one of those lazy people looking in and running off his mouth!
Money does not motivate me anymore. I am not the young man, I used to be! But I played the game well!

Thanks for thinking about me Art, it helps to feel connected.
I have Allstate, do you? (:)

Phil