Two valves in particular come to mind. Most likely it's the small vacuum relays mounted under the nose, just in front of the hood. They seem to be the most common culprit. Replacements are available but as others have already stated, they're iffy. I rebuilt mine about 12 years ago and they still function fine. It's not exactly easy and you run the risk of making it unusable but it is possible. Remove the valve and gently pry the ring that holds the diaphragm together. You have to do this with the idea of recrimping it in mind, so no cutting! Once that's apart, the entire valve will just come apart. The problem generally is the rubber seals on the stem. If the diaphragm is ruptured, well, you're screwed. The rubber seals tend to get stiff and don't flex anymore. So what you want to do is find a couple o-rings that are the correct size as the inside diameter of the tube, then gently use an Xacto or similar very sharp knife to cut a small groove on either end of that stem, just deep enough the o-rings sit in securely. Use a light grease like a lithium grease to aid in sealing and reassemble. Recrimp the ring around the diaphragm and off you go.
The other one is the seal on the headlight stem, which is also probably gone on yours and you should replace it also, but it's not likely to cause the problem you're seeing.
