I know nothing of you're tractor but I would be looking for a suction leak. Bad O-ring, cracked tubing, loose fitting, etc. Are you using the correct oil? Some oils foam worse than others.
Foaming fluid is caused by suction leaks as Kim wrote-you often do not see any external leakage since when the tractor is not running there is no negative pressure on the suction lines. Filters or the hose the runs from the sump to the pump are usually the cause.
If you keep running it that way, you risk damaging your pump.
Are you seeing foam on the dipstick? How much foaming and where are you looking at the fluid? Do you have normal power and function of your 3PH? Also, are you sure the fluid is not a bit milky from moisture rather than foamy?
I'm just asking all these questions because some foaming of the fluid is normal in the reservoir, but the fluid should not be full of air bubbles. If you rebuilt your hydraulic pump or put a new one on, did you change out your hydraulic fluid with new NH134A fluid? Those old Fords could easily have water leak into the transmission and rear differential or get condensation over time. You might also see a few air bubbles just because of turbulence in the transmission and differential casing. What might be the best indication of hydraulic pump and overall system health is how well your 3PH lifts and if it will hold a load when the tractor engine is shut off.