Technician A says that when a supply reservoir is completely drained, air pressure in the primary circuit should drop to zero. Technician B says that when the secondary reservoir is completely drained, pressure in the primary reservoir should drop to zero. Who is correct

None of the Technicians is correct.

Explanation:

Normally, when a reservoir supply is completely drained there is always some form of air pressure inside as the pressure that drains the contents may have been reduced but not entirely dropping to zero. On the other hand, the secondary reservoir when drained does not entirely affect the primary reservoir pressure also. Thus both assumptions of the Technicians A and B are incorrect.