Wilton Bench Vices

Another school has eliminated their Industrial Arts Program and I was fortunate to buy all of their Wilton bench Vices. They are all 4×10’s, all have the sliding dog or dog points, quick release and work perfectly except for two. Their problem is the dogs when slide up, just fall back down. Can anyone tell me, what provides the resistance to maintains the dogs raised position. How is the dog removed to inspect/repair? My first thought was the pin that sticks out the front, simply unscrewed and the dog would slide up and out. So far , I have not been able to remove the pins. Anyone familiar with the Wilton and can provide some advice? I believe they are 1950-1970 vintage, but that is a guess.