Power steering noise shortly after startup

I replaced the pump on my 96 2.5 with one from an 89 4.0 recently. It worked great for a couple of weeks, but then started to make noise intermittently. Well, now it starts with rattling noises, then what sounds like full blown cavitation. I replaced the return line because it was spongy, but to no avail. I'm considering replacing the pump, but the only other one I have is on a 90, buried in Renix brackets. Could it just be a relief valve? I can crimp the pressure line and hear it blow off. I just did it cold, ad before I could blow the relief valve four times, it was whining like mad. Still blows the relief valve and lugs the engine when crimped.

The fluid is normal checked cold, but foams before I can get around the block. It's not venting it off, the level remains the same, but it sure sounds terrible.

Try bleeding it again.
Do it by jacking the front wheels off the ground and then spinning the steering wheel back and forth, lock to lock, about twenty times.
Many times, you need to bleed the system multiple times to get all the air out.

Pump swapped and bled, sounding better already.

Well, it was as quiet as I could hope for. Sixty miles later, same as before.