I have a sargent crane20 ton  on a ccc carrier from the 60s with low hours, she was running fine all of a sudden  it stuck in boom up  . I cant see any thing  out of place, the band on the boom hoist is applied ,why? all the linkage looks ok,  I admit that I don't understand  this  system   .no  master clutch  just a hydo pump off the engine. thanks  Steve