I investigated the matter of how to remove silicone oils from vacuum apparatus rather thoroughly while writing the section on Diffusion Pump Oils in my book on "Vacuum Methods in Electron Microscopy". Here is what I was able to come up with (p. 185): "Silicone oils themselves are also very difficult to remove from the parts of an electron microscope because they are so viscous and insoluble. Some instrument manufacturers have refused to issue service warranties on instruments if silicone oils are used in the diffusion p[umps. The cleaning procedure recommended by the Dow Corning Corporation (I called them and discussed the matter personally with one of their engineers) consists simply of wiping away as much of the oil as possible with a dry cloth or tissue, followed by repeated wiping with cloth pads moistened with toluene, xylene, trichloroethylene, or perchloroethylene." In addition to the above solvents, kerosene is sometimes recommended. Repeated rinsing with one or mopre of these solvents may ultimately get your system free enough of the old oil for your purposes. You may want to check with the manufacturer of the pump before changing the type of oil you use in it, to be sure Santovac will work in it satisfactorily. If it was designed for use with DC-704 silicone fluid, it's heater may not put out enough heat to give a suitable rate of boiling with Santovac. If it uses DC-705, this will probably not be a problem.

P.S. you can order my book from Portland Press, Ashgate Publishers, Old Post Road, Brookfield, VT 05036-9704 (Ph: 800-535-9544) - it contains loads of just this kind of practical information.
I also ran across a detergent that is advertised as being suitable for removing silicone and other oils (p. 72). This is a product known as RBS-35 manufactured by the Pierce Chemical Co., P.O. Box 117, Rockford, IL 61105 (Ph: 815-968-0747). This stuff may be intended primarily for use on glassware, and so may require treatment in a boiling solution, I just don't recall; however, you might contact the manufacturer and see if they can suggest a method of using it on a DP. Incidentally, there will be silicone oil all through the pumping line and the hose attached to the DP, as well as inside the evaporator, and so these parts will also have to be cleaned if you want to get the system reasonably free of silicones. Best of luck! W. C. Bigelow