A sight glass found protruding from the center hub of a wheel allows you to check the level of:

Question

Accepted Answer

B: Gear lube/hub oil. The sight glass is mounted to the end of the axle as part of the hub and extends through the center of the wheel. This sight glass allows you to check the level of the gear lube, or hub oil, in the axle.

A sight glass found protruding from the center hub of a wheel allows you to check the level of:

Indiana CDL Pre-trip Inspection Test Info