Just bought a D-68P-1 LGP dozer. Is in excellent shape for the age. The guy I bought it from just went through the finals and replaced the steering clutches. When the dozer gets warm/hot after pushing for an hour or so the steering fades out on both clutches... I am not familiar with these machines and hoping someone out there could have some advise on this particular machine. If it sits and cools down, then it works great again for a while.
We had a very similar issue last year with our D-65-7 and the hose on the oil pick up ended up being the culprit after a lot of mucking around with it. It was just letting in enough air to be a nuisance. Our symptoms were remarkably similar to yours. We ended up replacing the hose and the clamps just to be sure.
Interesting point kiwi, the day before the o-ring on the fitting between the hard pipe and hose that feeds the steering clutch pump blew out. The guys put a generic metric o-ring in, wonder if it is maybe sucking air in that spot, maybe not correct o-ring?
I'm far from an expert but if you've been playing with that, then I would bet anything you like thats the problem. Also, check the clamps on that hose, they may have been loosened to help access to change the O-ring. For the life of me, I never could understand (and, frankly, still don't) why it went for an hour before it started to disappear. Start it the next morning, and it's away laughing. One hour later, and you could almost set your watch by it, you'd be hopping around on one foot with sore toes after kicking the blade out of frustration when the clutch disappears...
I'd check the o-ring and the rest of the suction side and any filters. Cleaning out the radiator and any other coolers may help. It could be a worn pump issue.