I have not seen a 6HBE30 with "power assist steering."
Forward and reverse question: Do you have to "play with the throttle control to get movement? If you do then my best guess would be the Travel Potentiometer in the control handle.
This is easily checked with a DVOM.
With battery disconnected, and key off.
Remove tractor cover.
Disconnect plug from the PMC (programmable motor controller).
This is the "black box" on the left.
Set DVOM to OHMs.
Connect DVOM leads to PMC connector wires 14 and 16. (note: do not force leads into the connector. I have used a small paper clip, cut about 3/4 inch long, bend a loop in one end to hold the meter lead and the other end fits comfortably in the connector. Do not allow leads to touch each other.)
These wires 14 and 16 come directly from the Travel POT (potentiometer).
A GOOD READING : throttle at rest = zero or close to zero.
Slowly rotate throttle = gradual increase from zero to 6500
ohm.
Slowly rotate throttle other direction = gradual increase
from zero to 6500 ohm.
If ohms jump , skip, go too high or low then the Travel pot is bad.
REPLACE IT.
New pot will need to be soldered to the wires. When reinstalling it , make adjustment to get the above ohm readings.
Check throttle for smooth movement and return to off, correct as needed.