I've seen them on Van Hool model motorcoaches.

 

New York State Commercial Driver`s Manual: "The tractor air supply control on older vehicles is a blue round shaped knob. You push it in to supply the tractor brakes with air and pull it out to shut yhe air off and put on the tractor emergency brakes".
Finally a wannabie had a chance to post something revelant

Anyway I have my roadtest this wednesday, hope everything will go well.

 

ok the blue valve...

while todays trucks come with 2 valves the yellow and red, while if u pull the yellow (tractor) you release both vales,

if you pull just the red you apply just the trailer breaks, if u hold the red and pull the yellow you only apply the tractor breaks and not both sets,

this is exactly what the blue button did, since the older break systems didnt allow you to be able to hold in just 1 button.

 

Yes, I would agree with that last post. I'm not sure of the reason for the button, maybe so you have an extra step to consciously keep the trailer brakes off..?

If you spinout climbing an icey hill, often times your tractor brakes alone won't hold your unit on the hill while you chain up so you need the trailer brakes as-well, you may even have to throw a tire chain behind a tire to hold on the hill.

I've seen a 2000 pete and a 95 frieghtshaker with these buttons, one was a black button.

 

I always thought that was the "Lot Lizard That Won't Get Out Ejection" button...

 

What you're speaking of is an "isolated reservoir valve".

Those were usually black and mounted separate from the parking brake and trailer valve.

As was said, the blue button feeds both the yellow and red button with air, if the blue one is pulled out, the red and yellow will not function.

I have seen mid 90's trucks that still had the 3 valve setup.

Had a 95 Mack with the blue, yellow and red buttons.

 

We have an 89 Mack Superliner with the three valve set up. They are marked as follows;

Red-Emergency,

Yellow-Tractor and trailer parking,

Blue- Tractor only parking.

When you pull the Yellow button out, it sets the brakes on both tractor and trailer, yet the red button stays in. When bobtailing, the blue button is the only one that is functional.

 

Yah, sure, whatever you say :roll:

 

I've seen newer Macks with all 3 buttons as well. The one thing they had in common was they were all Canadian market.

 

Here you go Ace, right out of the Mack manual. I was wrong though, the Red button says Trailer Air Supply, not Emergency, so you got me on that one. :roll: