I need to know what I should use to flush the reservoir on a high pressure electric hydraulic pump?

You should check the manufactures instructions for your unit, usually using fresh fluid to flush is done, there are chemical products available, but using the same fluid for a small system is probably the simplest.

Flushing Hydraulic Systems

http://www.customhoists.com/pdf/OilRecommendations.pdf

EHC SYSTEM CHEMICAL FLUSHING PROCEDURE