Can't bleed air out of rear brakes

did you bench bleed the m/c?
does the new the new p- valve have air in it?

I would start with the m/c-- if bench bled-- then move to the p-valve you installed.

pump your brakes & crack the line loose going to the rear-- see if air is present there.

if not continue to the rear system-- are the bleeders pointing up?

if nothing else fill the m/c & open the bleeder valves on the rears & gravity belled them for an hours-- to see what happens . Lawrence