towing an M923A1

[Dixmotorpool]

having an M923A1 towed about 200 miles.. rear drive shaft removed, front axle off the ground.. with air supplied to the emergency and service glad hands by the towing vehicle and the parking brake on the floor between the seats down/off should the air brake button on the dash be pulled out... or pushed in..??. I don't understand the air brake button, push to override spring brakes, emergency only, pull out for Normal.??..

 

[M35A2-AZ]

having an M923A1 towed about 200 miles.. rear drive shaft removed, front axle off the ground.. with air supplied to the emergency and service glad hands by the towing vehicle and the parking brake on the floor between the seats down/off should the air brake button on the dash be pulled out... or pushed in..??. I don't understand the air brake button, push to override spring brakes, emergency only, pull out for Normal.??..

The spring brake button needs to be pulled out all the time (for Normal). It is used when you need to work on the spring brakes.

 

[nf6x]

The button on the dash should normally remain pulled out. My understanding is that the purpose of the button on the dash is to force the springs brakes off (assuming air pressure is available) so that the mechanical driveline brake can be adjusted. Otherwise, the spring brakes would hold the truck in place and the operator couldn't tell whether the mechanical brake is doing anything. Conversely, to test the operation of the spring brakes, you can lower the handle to disengage the mechanical parking brake, and then manually push up the little valve that the handle bumps into to apply the spring brakes.

I've accidentally bumped that dash button while working on stuff in the cab. Since it should normally only be pushed in while adjusting the mechanical brake, I'm considering adding a safety pin of some sort to it in my trucks.