Tha makes sense about the way the brakes are applied. I can confirm that with the power disconnected I can rotate each of the motors by hand, including the Z. The machine has belt reduction on all three motors too. I fully expected the runaways, I've got the motors off the machine and strapped down on the bench for testing. the drives are bolted onto a frame that included the DC power supply. It has the 3 drives with a connector on each for the diff signal, tach signal, and limit switch inputs. There is also power connection to each of the servos, Pins N and P. There is a single 24vdc controlled relay on the frame of the unit that interrupts the 115vac coming into the power supply. It appears as though this want meant to enable/disable the drives. After looking through the documentation I was sent from servo dynamics it does show that each of the drives has a pin to enable/disable it externally, however, it's not being used. I'm not sure If I should deviate from the "stock" wiring and connect these pins to an estop circuit or just rely on the relay that already there being controlled by the estop circuit.
Edit: the machine has a pneumatically actuated mechanical brake on the Z.