Interesting Brake Problem

"Lucille" decided to act up on me today as I was driving her around on some errands. I came to a stop light so I stepped on the brake pedal, and it seemed like the pedal kind of pulled itself down and locked the brakes. I wasn't on a very busy street so I just waved folks around me. They were really, really locked too! I tried to stomp on the gas in gear to see if I could at least force her to the curb, but nothing doing! So I sat there a minute and fumbled for my AAA card. But meanwhile I hooked my foot under the pedal and pulled up, and while nothing happened at first, after a minute or so, the pedal came up and I was able to move. I made my way home making absolute minimal use of the brake and then only with a very light touch. Once in my driveway I stepped on the pedal a bit harder and, sure enough, pedal lock. I was able to pull the pedal up and lock it several times to verify repeatability. So now I have to figure out what's going on and how to cure it.

Update: I'm pretty sure the culprit is the vacuum booster. It is the Moraine unit and was supposedly rebuilt before I bought the car, about 3 years ago. Now that underhood temps have dropped, the sticking pedal is harder to duplicate, but it will stick for several seconds before letting go even now. But, only when the engine is running! Switch off the engine and pump a couple of times to use residual vacuum and presto, no more stick.

Now to decide if I want to rebuild or try an re-engineered approach.