I have the front manual window reg out of the 65 to do some cleaning and roller replacing. I notice if I use the crank to run the reg thru its full travel, the spring winds and returns the mechanism to the full-up position. This can't be right! What is the purpose/function of the spring? I haven't taken out the pass side, but it doesn't do the same.

The spring is to aid in the weight of the window rolling up.

Maybe when it has weight back on it, it will be okay? Can't figure out why the pass side doesn't act the same?

You said you have the driver's side out, is that where the spring returns it while its out, or while its in as well. Mine didn't act like that in or out, but it did return with some force when i rolled it back with the handle. Maybe your pass. side is just dirty.