Question on Transcraft step deck

Large lever is NOT your ride height, or at least I doubt it is. Our trailers with a tag axle that lever controls pressure to the tag axle. So I'd guess it's to adjust pressure to an axle. If its just a tandem spread not sure why

 

The lever is to raise the bags for the rear axle to have enough clearance to slide. Turn one off the switch valves to activate the height control. Raise trailer for clearance, slide axle, turn switch valve to go back to leveling valve. If you have more weight on the trailer, you'll need more pressure to raise for clearance.
Make sure you keep an eye on that kingpin plate. Transcraft of that era are terrible for rotting out in the neck.

 

Probably an old ATS trailer. OP, if I were you, I would get this working ASAP. Closing the spread is much easier on the tires. Take a hammer and tap the pins. They should go in.