Your pump should be able to "pull" more vacuum.
Are you testing directly from the hard plastic line tha runs from the brake
assist to the vacuum pump? Take it loose from the master cylinder end +
hook up mighty vac ($25.00) for the plastic version @ autozone.I think the
vacuum pump is rebuildable.Tiny little valves sometime stick (spritz of brake
parts cleaner)Check with Phil @ the top of the page about parts.Vacuum at idlewill be different from some r.p.m.'s.

If your chassis does not have a separate pump for the door locks you may be
correct in your suspicion of the vacuum pump;and you might as well start
looking for a vacuum leak if the pump is O.K.

if you have a leaking master cylinder assist there is a sequence for "pump" testing it with your foot, I am unable to remember it at this time .

Some of the more experienced members will chime in I am sure!