I can't remember, so apologies if mentioned earlier.
Have you checked the pressure relief valve is seated correctly?
What oil are you using?
And what pressure are you getting when running at 2-3000rpm (ish) and idle, hot and cold
I have not had any issues with used 1500 pumps, they are indeed a higher capacity type, so preferable. Though I have also heard they are often out of tolerance when out the box..... A good used one may be a better bet.