What make/model/year? If GM in the last 30 years, open up and remove the filter/orifice. If clogged with aluminum, etc. , replace. If other make, others will chime in.
Vacuum for a looong time, hours, not the 30 minutes shops give it due to time constraints, overnight is common on systems that have been opened, heat lamp on dryer to make it release the moisture from the silica gel. Maybe 30 mins at a time for the heat lamp. Then does it hold vacuum overnight?