Air brake question for the heavy mechanics

[greywuuf]

I am trying to move an "H" series (7500) that does not run. I believe them to be "modern" "s" cam brakes.....there is a unlocking bolt installed on one side of the rear axle but when I went to install one one the other can .....I.can not get it to "hook" ....it feels to me like the diaphragm (or spring perch or whatever is inside there ) is so far in that I can not get the bolt through it.
I do not know if this makes any sense to anht of you or not....but never having seen the inside of a can (but knowing enough NOT to take it apart) I am at a loss as to what to do? Any tips hints or suggestions?