Interesting theory. I think it may be based on a false premise though. (Not that my theory is any less false, but it is an opposing theory). Your theory of the pressure cooker is based on (among other things) the fact that the voice and passion is given no out until radio came about. However, town hall meetings have held the same effect for many generations long before radio. Yet even with these outlets, there have been revolution. You point to communication restraints, as being the "stuck closed relief valve", but I contend that these actually stopped revolution rather than released it. By restraining communication, many revolutions were quashed before they started - they never got momentum because they were a bunch of isolated pockets easily divided and conquered. Because of this, it truly had to be widespread dissatisfaction to reach critical mass, which I contend was not because of, but rather in spite of, the communication restriction.
Radio (and other "new" media) on the other hand do not restrict communications, or at least not as much. Therefore dissatisfied constituencies can take a "running poll" on like-minded individuals. I still believe revolution would occur if the critical mass were reached, but what radio and other media outlets provide is communication that leads to organization. Because of this, any critical mass would occur MUCH quicker and hence would seem to arise overnight in comparison. I think the "relief valve" in this theory is actually the opposition also seeing the results earlier and changing course before being overwhelmed.
Hope this all makes sense, it is a 5 minute work break at 3:30AM and I am really not very coherent at the moment.