CM2350 fuel rail issue

[ktblogistics]

have another truck this time with 559 code
did rail leak test first, dropped 5k within 1min
with tool to fuel return manifold
-no drops on relieve valve
-on high pressure return line 300ml within 30sec
-on injector return line 50ml within 1min
cleaned high pressure pump inlet line banjo fitting with shop air

dont know how to test low pressure fuel side , have quickserve account

will have a truck back somewhere next week back, what else i can check
when fault code set up think commanded pressure was 34k measured 30k
fault count 640
all help is appreciated, thanks