Complex sentence: The twin spherical iron air bottles were charged with 900 psi air to power the automatic pilot ("Steurgerät"), and at 90 psi, the pneumatic servo-motors for the elevators and rudder.
Split into simpler sentences: The twin spherical iron air bottles were charged with 900 psi air to power the automatic pilot ("Steurgerät"). Air at 90 psi powered the pneumatic servo-motors for the elevators and rudder.