I set up a test rig to check gauges and senders. I use an old porta-power pump with a T manifold and a calibrated mechanical test gauge then connect the unit to be tested to the other port. I have an old Ford King Seeley indicator gauge known to be good that I use to test the KS senders. Connect everything with jumper leads and a 6-volt battery then apply light hand pump pressure to the manifold & bleed it before testing. The mechanical gauge and electric indicator should be within 10% of the same reading as the test gauge. A person can also test the indicator gauges as well using a known good sender.