I had the same problem with my Vacum pump and since my car is a beater and I do not want to put any money into the car that I do not have to( I know it is destined for the boneyard soon) I try and disassemble and fix whatever prior to buying a new unit. I did this with the vacum pump and cleaned out the one way valves, there are 2 of them and the pump works fine now. If you are going to remove the pump anyway take it apart and clean the valve and reinstall. You maybe as surprised as I was. Saved the cost of the pump. Will have to see if the valves last another 300k miles.
John