57 3100 brake pedal stuck at top?

[tmoble]

So I'm driving home from work in my (owned and driven since 1983), I'm rolling up to the guard shack at the parking lot exit, hit the brake to wait for the arm to swing up and the brake pedal is like stuck at the top. Doesn't hardly move at all. results in the pic.

It's a stock M/C and pedal setup. What would cause this? is there some way for the linkage get over center or something? It's pretty simple. It's dirt simple. Truck still in the parking lot at work, get a wrecker in the morning.

I've rebuilt that M/C twice over the years, but I'm clueless.