I have experienced way more problems with rubber vacuum elbows rotting away into nothingness and even vacuum modulators taking a dump. Make sure that all of your vacuum lines and elbows are in good order and hooked up properly. If that is all good then start the car and take the two lines off of the modulator and touch the open ends together. This will effectively by-pass the modulator and apply vacuum right to the valve. If the valve is working properly the engine will stumble, if not then you need to take a look at that valve and see what the deal is. It could be a plugged valve, plugged pipe, or possibly a torn diphragm. If it does stumble and you have a code posted you may have a problem with the temp sensor or the modulator itself. I don't have my blue book in front of me but I believe it is mode 3 on the A6 hole that will put it into activation mode. While in activation mode it will run that modulator and you can feel it click. If you hook up a vacuum pump with a guage on it while it is in mode 3 you can see the guage fall everytime the modulator opens. Being a smog technician I have seen all of these problems but I have yet to run across a plugged valve or pipe in a Volvo, but as these systems get older and older you never know what you're going to see.

Mark