I was afraid you might ask. Basically you need fluid at the pump intake.
We always run the pump output into our gauge. It will peg a 5000psi gauge in about 3 seconds, thus its a two person job; one to stop/stop the motor and one top verify. If we don't have pressure then we worry about priming. The easiest way is to stick an air blower into the reservoir sealed loosely with a shop rag. Gentle pressure is placed on the fluid to help gravity flow the fluid into the pump. Periodi8cally we crank the motor till we have pushed fluid to the output.
This is very seldom necessary.
Bosch Master, ASE Master, L1
33 years MB technician