several faults can cause the bas light to stay on.

what happens when you turn the key to position 1? does the pump run for a little while and then go off? if so, pump is ok and pressure switch is ok.

if you go straight from key 0 to 2, does the pump run as on key 1, and what happens to the orange light? does it go out when the pump stops? if so, and the light only comes on again after driving, you have a sensor fault.

if it stays on at this point there are several possible faults. sensor fault, ecu fault, level sensor fault.

my guess is that during the engine rebuild you have accidententally unplugged or disturbed the connection to the fluid level sensor on the reservoir (the flat 3 way connector). check it out.