Mike,
Here is a paragraph in the manual - I think it has to do with steering:
Vacuum diaphragms are used to engage the rear steering rod to the steering logic assembly. This allows steering wheel movement to be transferred to the steering pump swashplate arm, controlling the speed and direction of the steering motor inside the transmission. Vacuum for the diaphragms, one left and one right, is provided by the engine intake manifold.
Here is a paragraph in the manual - I think it has to do with steering:
Vacuum diaphragms are used to engage the rear steering rod to the steering logic assembly. This allows steering wheel movement to be transferred to the steering pump swashplate arm, controlling the speed and direction of the steering motor inside the transmission. Vacuum for the diaphragms, one left and one right, is provided by the engine intake manifold.