when you hold it on the lock, turned hard one way, does the pump work at all? does the hose get pressured up and try to jump/move or straighten out some to be in a straight line under pressure and then when you let the steering wheel turn a bit the line slacks off some (the system will go to about 1500 psi so the hose will likely move if the pump is putting out)? if not it could be a poor pump or a bad relief valve. have you looked at the valve and possibly the fitting screwed into the valve to ensure everything is right?
here are a few links to some info. the first and second are videos explaining how to test the pressure in the system. the third explains how a power steering box works. the rest are just info on relief valves etc. hope it helps. its for a ford but most will work the same.
https://www.youtube.com/watch?v=h9rgkZ_R-Mc
https://benchworksteering.com/tech-t...-relief-valve/
https://www.youtube.com/watch?v=AeXj35aOK7k
http://www.gmtruckcentral.com/articl...rsteeringmods/
https://www.hotrod.com/articles/ccrp...ering-secrets/
http://www.corvette-restoration.com/...-pump-rebuild/