The problem with the STV is that they are vacuum operated (via the vacuum modulator, which is controlled by the temperature lever), and the diaphragms a) tend to go bad, and b) are expensive to replace. Luckily, the failure mode in the Riviera systems is full cooling; in the full-size cars a bad diaphragm means minimum cooling.

If you're mulling a change to R134a, consider this: you can retrofit for well under the cost of a full R12 charge. The worst part of the change is draining the compressor and replacing the mineral oil (incompatible with R134a) with ester oil. This will only cost you $5 and some time, and the ester oil will work with R12 if you decide to switch back.

To make the change:

• Replace compressor oil.
• Install new ports for R134a fittings.
• If your system hasn't been working for some time, replace the receiver/drier. You'll want to do this even if you're sticking with R12.
• Evacuate system.
• Run your engine @ 2000 RPM, full A/C, with an auxiliary fan blowing across the condenser.
• Install 75% of a full load of R134a (if your system uses 4# of R12, start with 3# of R134a).
• Add R134a 1 oz. at a time until you reach max cooling at the outlets (stick a thermometer in a vent). When the temperature starts to go up, remove what you just added.
• Adjust the STV (loosen the lock nut and turn it in/out) to get an evaporator pressure of 18-23 psi on an 85 degree day.

NOTE: you will see bubbles in your receiver/drier with a full charge of R134a. This is normal.

Properly installed, this should get you to within 3-4 degrees of an R12 system. When you consider the original cooling specs, that's compatible with (or better) than some new cars. Of course, you will benefit greatly from a fan shroud and an operational fan clutch.  If you're going to drive in stop-and-go traffic on a regular basis, I would recommend an auxiliary electric cooling fan in front of your condensor. It might not be stylish, but neither are sweat rings in your armpits. You can fit a fairly large fan behind the grill on those cars. Hook it up to the A/C clutch switch, and you'll help yourself out every time you drop below about 35mph.

There are also rumors that R134a is more likely to leak through hoses due to its different molecular composition. I don't know if that's true or not, but there are two countering arguments:

• The hoses are so thick that, while technically correct, it's not a practical issue.
• Even if it does leak, it's a slow leak, you can replace a lot of R134a for the price of new hoses. It's like replacing your main seals to stop a slow drip when oil's .69 a quart.

Free advice (you get what you pay for): if your system works now, stick with R12. If it doesn't, make the low budget switch to R134a.