Blue dash button COE tractor

http://www.michigan.gov/documents/Section_5_-_Air_Brakes_109900_7.pdf
Older trailers did not have spring brakes and would have no brakes on when parked . Applying air set the brakes .

 

I have wondered about this for some time also...why would there be all 3?

 

The red is trailer, blue is tractor, and yellow is both. These older trucks also do not seperate brake lights and signal lights on the trailer. Brake lights will illuminate the side blinkers.

 

some old vehicles had addtional button - park brake valve air saupply. It pops out when park brake tank air pressure is lower 75 PSI.

Now this button is used on Scandinavian trucks. They are equiped with combined European park brake valve (one stick, no buttons) and this button valve. If truck loose air it pops up and park brake valve wont working till you push this button.

DOT and ECE do not require it, but Scania and Volvo use this valve

 

Last year a QC driver had a trailer come unhooked so corporate safety e-mailed every driver instructions on how they taught us to hook a trailer . Their instructions said to put the trailer parking brake to "emergency" to apply air to the trailer brakes .
I replied and asked them to send more detailed instructions , preferably with photos because I couldn't find the trailer parking brake they described . I never got a response .

 

Trailers made after 90s got all PREV valve. If you disable air supply (red button in off position) ore red glad hand separated trailer brakes are locked. some trailers got buttond to apply/disable brakes (red and black buttons), but ther work if trailer air tank is not low. On some trucks, like Volvo FH and VN, some Freightliners there are trailer hold switches (electrical ore air). If you turn trailer hold on blue/yellow (service brake) glad hand is pressurized. But never use this button for parking - pull both red and yellow buttons.

It is not all, some old trailers got no PREV and spring brake. They used chocks ore trolley brake!!!!!! I advice everyone to convert air system of old trailers to new DOT and ECE requriments

 

On my 78 freightliner it was to release springs if I lost air there was a separate air tank to allow you to move the truck if breaks locked up