I have a newly acquired 1978 JD 750 (E or B?) Dozer. After warming up to operating temp in a hard push the transmission pressure gauge will drop into the yellow and it quits pulling and just chatters.....the engine is in great shape with only 1000 hrs since major....undercarriage is in great shape.....it is worse if on a hillside vs flat ground....it doesn't always do it and only when pushing hard. As long as pressure stays up it is fine....Any ideas? Thanks, Jerry
1978 would almost surely be a straight 750. You need to start with checking the hydraulic fluid that it's the correct type and in good condition, charge filters, suction strainer if any. Does it lose power pretty evenly on both tracks? Do you have the rabbit/turtle control in the slowest setting when pushing hard? Probably badly worn motors but there are LOTS of things that can cause loss of charge pressure. When you lose charge pressure the pumps can't stay on stroke, that's what moves the swash plates typically.
I had similar problems with my 750. Deere mechanic recommended we drain hydraulic oil and replace with 30 wt non-detergent. A big improvement. BTW, my 2012 750J hydraulics runs engine oil per factory specs.