Warren,
Just in case this applies:
The threaded shaft must have a locked surface to pull against. Thus, the allen screws must tighten something somewhere. Just screwing the rod w/o a solid surface to pull against will not move the vac cap inside.
BTW, Frank wanted you to pull the rod out and see if there is a hard pull proving a vacuum still inside.
If this is redundant info, then sue me...
T