I’m really not crazy about the 600 side pull brakes. I have the same problem with the quick release... just seems like it’s not the best design. My rear one has worked its way open once or twice. Brake still works with it open but it takes a lot more travel from the lever before it actually grabs the rim. The early Shimano levers (these) look really cool, though.

Since I’ve been riding the AD to work I’m probably going to find something to swap them out for that is better but period-correct. (If there is such a thing.) Don’t want to have crappy brakes while riding past a bus terminal.