How to help a yellowed brick with a sticker? My parents recently finally decided they can return me my childhood toys with 1990s Technic sets including the 8880 Super Car. All other sets were in boxes, but this one spent the last 15+ years assembled in a clean attic, so it's dusty and yellowed, but surprisingly, complete (I disassembled it with the instructions in hand). I haven't yet decided if I should keep or sell it, but I need to clean it.

Question:
This piece.



I've read about bleaching LEGO bricks with hydrogen peroxide and about removing labels for repair. Do you think it's a good idea to remove the labels and then bleach the brick? Or is there some other, better way of helping this guy?
A paste of oxygen bleach slathered on liberally, and then some time in the sun will probably fade the yellow nicely. It should not degrade the plastic (any further).

Your real problem is not re-attaching the label, but removing it without tearing or leaving behind most of the backing.

Depending on the material and glue, you might be able to remove it carefully with water, if the glue can be dissolved in water. If you can get the paper off and let it dry flat, you should have little problem re-gluing it. 

The challenge will be the choice of glue. You need something that isn't necessarily permanent when it cures, but can still stick to plastic. White craft glue (not yellow PVC!) should do the trick. But there are many modern adhesives used for non-permanent applications that can bind plastic and paper. There might even be an archival "spray-mount" style compound that won't yellow over time.

Though, honestly, white craft glue is pretty stable. You can even make a paintable slurry from white craft glue that you can apply with a paintbrush to one surface to make the whole thing easier.

But it will be removing and preserving that labels that I suspect will be the greatest challenge.