The right auxiliary pump and the xfer unit were switched off at cruise; FL360. At top of descent; the auxiliary and xfer unit pumps were switched back on. Hydraulic pressure and quantity were normal. Between FL240 and FL220 and approximately 60 NM north of ZZZ2; the master caution light illuminated and indicated left/right hydraulic pressure low. System display panel showed hydraulic pressures zero and quantities L19/R9. The first officer began working the non normal checklist for hydraulic pressure low in the QRH. This particular checklist ensures all pumps are on and then in an attempt to remove air from the system the checklist requires the engine hydraulic pump and auxiliary/xfer pump be switched off and then back on. The first officer accomplished this 4 times with no permanent rise in hydraulic pressure ....