For years my dads 955 would intermittently lose steering due to the booster pump losing prime. I'm still not sure the reason, but have tried to eliminate all possible causes short of digging into the rear clutch housing to find out if there's a bad seal or a rust hole in the suction pipe. I've nearly doubled the amount of trans fluid, with no change in performance. Any ideas that I haven't tried, feel free to shoot them my way.
You hit on the most logical, sucking air. My Allis was sucking air from the pump shaft seal.
My latest attempt to solve the issue is to connect a suction hose under the foot board where the prescreen is. The boss is gonna connect to the pump suction and run to the back of the machine under the battery box where the left side brake access cover is. The cover is gonna have a suction pipe through the cover, and extend down into the case sump, all stainless welded joints. Hopefully this will be the fix. And it's tucked up out of harms way of tree roots and rocks.
Got it put back together today. Fired it up and first thing I noticed was I could feel the oil being drawn through the suction hose. Let it idle for a few minutes. Then put it into forward, stepped down on the left steer pedal. It turned left.did the same to the right, forward and reverse. Woohoo!!! After 30 years of intermittent steering, and having the Cat dealer trying to sell me a rebuilt booster pump, it steers.
Excellent. Was just sucking air, May be worth going into OE style system next chance and returning all back to original configuration.