Red button

It’s a throttle. Turn the black knob to ease rpm’s up. Push the red knob in to release the throttle. If my memory is correct. Been a while

 

Thats what we used for cruise control before cruise control existed.

Its a hand throttle... Does that truck have a PTO? If so thats most likely why it has a hand throttle.

 

Must be a mechanical engine. Two of ours have the same hand throttle.

 

Yep it’s a manual throttle cable. The truck likely has a PTO and no electronic cruise. Had one in my old Pete with a 3406B mechanical engine.

 

Say What ???? What's this cruise control you speak of ???

 

I'am just waiting for old @Dino soar to see the OP's avatar then see his picture he posted and then his 4 page novel reply about Peterbilts ,train horns, and belt buckles

 

It’s the “Ejection” seat button when the blinker fluid button isn’t working……..

 

You've GOT to be kidding,,although, I shouldn't be surprised, it's one of the few mechanical, manually operated devices on a truck. Came out in the 60's, "Polish Cruise Control", ,I'd be more curious what that valve and plumbing does?

 

Train horn