1938 Buick Century Fuel Sending Unit...

[Bloo]

It isn't normally 100% free by the way. Do you see the little spring on the shaft down by the gear? the spring pushes on a brass washer, It is keyed to a slot so it does not turn. Under that is a cork washer. and under the cork, the gear. Those parts acts as a brake. It means in a nutshell that the reading doesn't change as easily as it could. It makes the gauge more stable, but the reading doesn't change much unless you are driving.

 

The brake probably doesn't work anymore 80 years later. If the brake doesn't work, the gauge will still work, it will just wave around more when the gas sloshes in the tank. I don't believe the reproduction senders even have a brake.