Rebuilding the mystical
 Suction Throttle Valve (STV)

The Suction Throttle Valve was used by General Motors in the early 60's as a way to vary the conditioned air's temperature at the evaporator. This method was used instead of cycling the compressor like most modern cars do. I actually prefer this design on a vehicle without an idle air control circuit, as it allows you to tune your car for A/C and not have to deal with RPM fluctuations while the A/C is cycling. 

Here is where I start. I know the diaphragm is leaking because when I pressurized the system (using a special made adapter and dried shop air) I could hear air escaping out of the holes of the vacuum actuator. The holes are on the system side of the actuator, not the vacuum side, so that sound spelled trouble. The rebuild kits will tell you to remove the STV from the car, but in my situation neither fitting had been removed from the STV in its 41 years of life, and after a little trying, I knew the fittings were not coming off without a cutting wheel. Seeing how they did not leak, I made the decision to rebuild the valve on the car. I would bet the valve got just as clean on the car than it would have off, but it sure does hurt leaning over a fender for half a day.

After removing six Phillips head screws, the two halves of the STV can be separated. Note: Unscrew the vacuum actuator beforehand and keep a firm hold on it, as it is under spring tension.

The piston. I assume the nasty gunk on the old diaphragm is old compressor oil.

The aftermarket replacement is made from the same material, rubber, as the original. When I first felt the original I thought it was a type of tar paper, but nope, it was just gummed up. I purchased the kit from Classic Auto Air Manufacturing after seeing a listing on eBay. They run about $45 plus shipping. Old versus the new. The new one comes with a "teet" in the middle which was not present on the old one, but it looks like it could have been there and disintegrated.

Aside: I chose to go the route to save the original function of the STV. There is also a kit out for around $100 that eliminates the function of the STV and converts your car to a clutch-cycling system. While you would never have to worry about a sticking piston with that kit, I preferred staying with the original function because I dislike clutch cycling. But if you are investigating going with the kit, here is how it works: You pull out your piston (the red part in my pictures), and between the two halves of the STV a metal block off plate is installed. A ball bearing is put in the vacuum hose and put back on the vacuum actuator for cosmetics. Then, a thermostat is mounted to the exterior of the evaporator core which is spliced in to the compressor clutch lead. If you don't have a show car and have fuel-injection with idle air control, this method may be a good solution for you. 

The new kit comes with a selection of O-Rings. I found this to be a great enclosure, as my large assortment of O-rings from the parts store did not have the small size needed by the oil bleed line.

Piston after being cleaned and diaphragm installed.

Here is everything cleaned and lines up as it will go back on the car. Pay special attention to all the rare, miniature pieces that can roll off into oblivion.

The cleaned other half of the STV. This took about an hour to clean with rubbing alcohol and paper towers. I love how you can open up A/C systems and transmissions 41 years later and they still look as great as the day they were built.

Piston, etc, reinstalled. Notice it is dark now!

Everything back together. Did you notice the newer, shinier vacuum actuator? My old vacuum actuator was shot and would not hold a vacuum. On a Riviera, this would not have been a problem, as their STV is designed so that no vacuum equals maximum cooling. On the big cars, however, no vacuum means minimum cooling. I will have you know these vacuum actuators are not a common item anywhere, including online stores and specialty supply houses. I did finally manage to find a company that had some, The Part Guy. He rebuilds STV so he has an extensive line of parts. I believe I paid around $80 for the actuator. These are factory originals, meaning good, used parts. Without this, I would have most likely had to go to a clutch cycling system to get good air conditioning.

Since I took the old vacuum actuator off, I took it apart. Let me tell you there is no way to rebuild the diaphragm inside it. If you could machine a generic diaphragm onto a new connecting rod, that might work, but the only two choices out there for vacuum actuators seem to be The Part Guy and junkyards with 60s GM products in them.

Once the system was back together, it was time to do a leak check. I used the guts of a Harbor Freight vacuum pump to make a shop air to Schrader fitting adapter. Note I used cooled, dried shop air to pressurize the system, as I did not have Nitrogen. Results: STV no longer leaks! Now I have no major (audible) leaks, but I still have to take the Buick down to an Air Conditioning shop and go through it to find the minor leaks. Once it holds pressure I can let the shop add some R-12 from my 30 lb. container stash, and see if I have AC!