Hard power steering after a few minutes

At our yard we've got a 1967 Ford 3400 Tractor/Front loader. 4 cyl Diesel. I'm having power steering problems. A line blew going to one of the assist cylinders and I lost all the fluid. I did a little research and was told to use New Holland hydraulic fluid in the P/S. When I run the tractor now, the P/S is easy for the 1st few minutes, then hard as a rock ..... I opened the P/S reservoir and found that the fluid was foamy ... I'm assuming the foamy-ness is what's causing the P/S to get hard ..... Am I supposed to bleed the system? or is it self bleeding when you turn it lock to lock like in a car or truck? If it is self bleeding, I did the repair about a week ago, I would've thought the air would be bled out by now ... Should I be using something else for P/S fluid? Like Ford Type F ATF? What do you guys use?