Hay, WD ....
Until one of the 'seasoned vets' can explain it better, here's a simple explanation. It's 'kind of' like the 'emergency brake' lever on a PV. It engages the 'spring brakes' on the TRACTOR ONLY ... if said unit HAS such brakes. No air involved, as opposed to the red & yellow knobs.
The ole' Pete Tom drove hauling asphalt had this such knob.
Hope that helps (a tad bit!)
~ Anne ~
I remember those as a kid when my dad used to bring me to his job to sit in a truck..
So I asked him and he said it held the foot brake while you check lights and for air leaks.
New! Check out our help videos for a better understanding of our forum features
I can across a CMV photo of three knobs
Tractor brakes Trailer brakes
And Spring release.
Is their an experienced trucker that can shed knowledge on this third knob? Is this from trucks of past? Special application?
Inquiring minds like to know.
CMV:
Commercial Motor Vehicle
A CMV is a vehicle that is used as part of a business, is involved in interstate commerce, and may fit any of these descriptions: