Air carrier Captain reported an aircraft refusal due to multiple hydraulic system deferrals.

Narrative

Flight did not operate ZZZZ-ZZZ due to MEL 23-XX which had a deferred Air Driven Pump (ADP) and deferred Center Hydraulic Motor Generator (HMG) with MEL 24-XX.Due to the center ADP and HMG being inoperative; I determined that it was not safe to operate in the event of an additional system failure. While discussing the plan of action with Dispatch; I was conferenced with Maintenance Control and spoke with Maintenance Controller; who requested clarification regarding my decision to declare the aircraft not suitable for flight. I explained that the decision was based on a reduction in redundancy to a critical hydraulic system. During the discussion; it was stated by maintenance controller that the aircraft would likely be down for several days and that repositioning it to ZZZ; a maintenance base; might be preferable. Additional comments were made regarding the age of the Boeing 767-300 fleet; the use of refurbished components; and prior operations conducted by other crews with similar deferred systems. These comments did not address the specific safety concern raised and were not directly related to the criteria.After I reiterated that the aircraft remained not suitable for flight; the conversation concluded; and Dispatch and I were aligned on the decision. Over the following two days; my crew and I reported for duty and remained at the airport for up to five hours each day while Maintenance Control continued troubleshooting the aircraft. During this period; I made multiple attempts to conference with both Dispatch and Maintenance Control to obtain updates on maintenance progress; however; each request was declined and I was repeatedly informed by Dispatch that Maintenance Control was unavailable to speak.