Not too long ago I was eyeballin an old military trailer that had an air/hydraulic combo brake system. There was a QD fitting on an air hose hose by the pintle eye that ran into what looked alot like a hydrovac can, just smaller, with a master cylender on it that pushed the wheel cylenders. That might be what the lever is for, manual trailer brakes, check the back of the truck for a hose connection. Of course tracing the hoses coming off the lever system would work too. smile

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P