CAT 3126B no start

[Aussie Steve]

HUEI pumps are well known for failing without warning as well. They have had more updates than I've drunk cold beer. Solenoid failures, high pressure pump failures - the list goes on.
Test the injector actuation oil pressure, check the reading with ET and compare to actual, cranking pressure should be above 870 psi with ET. gauge in the test port in head with everything left as is so the engine can run and fire. Confirm with a manual gauge. If the injector actuation pressure is below 870 psi your HUEI oil pump is possibly shot, it's exchange only no serviceable parts available. You can also remove the high pressure oil line to the head and install a gauge in the pump outlet, pressure should be a minimum of 4000 psi. That will confirm if it the pump or the IAPCV valve.
And if all else fails check the drive gear isn't loose on the HUEI pump, that's a possibility with a freshly rebuilt engine. Oh and the engine oil level is correct.