Check that vacuum is getting to #23.
If so, #23 may be bad.
If no vacuum at #23, check the vacuum lines (not shown) running between #23 and #16 and from #16 to engine for leaks.
If vacuum is present at #16 but not at #23, #16 may be bad or not receiving a signal to open.
Is there another page regarding the PTO? Most of this hardware is described as 'Blade Brake.'