DMV Strikes Again!

The yellow is tractor only red is trailer only. Trucks are not equiped with the blue knob any more but if you had one if I remember correctly the blue one releases truck and trailer spring BRAKES

 

Page 5-4 of this .pdf linked manual will give you the answer. It also goes briefly over the ABS light checking from the Michigan 2005 Commercial Driver's License Manual

Here's the picture from it, but read the manual description especially the ABS Trailer light checking.

 

"Brakes." Not "breaks."

Red octagon: Trailer only. You would pull this if you needed to slide a trailer tandem. It's not considered a parking brake because it doesn't set the tractor brakes.

Yellow diamond: Tractor and trailer(s). Parking brake. Sets all brakes.

Blue round: Tractor only. I don't think these are used in newer trucks. The yellow knob is good enough. But you might still see a blue knob from time to time.

 

See they say you can't teach an old dog new tricks. I couldn't remember what brakes the blue knob set. Even when I drove a truck with them I don't remember using it more than once or twice

 

Red knob controlls trailer spring brakes.

 

I've always found the air brake-spring brake-service brake terminology to be 1) old fashioned and 2) confusing. Brakes is brakes; you got yer drums and shoes or discs and pads.

Use brake foot pedal (aka service brake) = apply tractor and trailer brakes, like when you're driving; you don't fully set the brake springs, which would hold the shoes against the drums. Let up on the foot pedal, the brakes release, and you roll.

Use trolley lever = apply (or test) trailer brakes but not tractor brakes; you're testing the trailer service brake circuit (yes, and the brakes themselves) and maybe the fifth wheel connection--you'll learn about that later).

Pull the yellow knob = set tractor brakes. (I said tractor and trailer up above. On the trucks I drove, if you pulled the yellow, the red also popped out if you didn't hold it in.)
Also known as spring brake/parking brake. Not the service brake circuit. Push the yellow knob in, you fill the pots, compress the springs, and release the tractor brakes.

Pull the red knob = set the trailer brakes. Also known as spring brakes. You're not using the service brake. Instead, you're fully exhausting air from the pots and setting the springs. Push the red knob in, you fill the pots, compress the springs, and release the trailer brakes.

The air brake-service brake-spring brake terminology refers to how you apply the brakes. The yellow and blue knobs do the same thing for the tractor: they set the tractor brakes by exhausting air from the pots and allowing the springs to set up--and that is what the red knob does for the trailer.

I always thought of it this way:
Yellow parking brake = fully activate spring brakes = set brakes.
Red knob = air to trailer to release brakes and (pulled out) no air to trailer to set brakes = fully activate spring brakes.
Service brake = foot pedal
Trolley brake = trailer test/tug
Air brakes = the whole thing

Shadow -- I'd guess the DMV answer is that the red knob controls the trailer spring brakes. (Unless you pop the yellow knob and the red knob also pops out. Crazy, ain't it?)

Also -- "setting tractor brakes" doesn't apply to the front axle. Tractor front axle brakes are service brakes only. Okay, there may be some applications where that's not true.

Also -- Red knob "not for parking." Hmm ... used to be that on parked trailers the brakes sometimes would release because air was used to set the spirngs; the air would leak out. Modern trailers are the opposite: you have to fill the system with air to release the brakes, which is much safer--a trailer could stay parked for years. So that may be why the red knob says "not for parking." In practice, when you park a trailer, you set the brakes = activate the spring brakes.

 

yeah but what does the plaid button do?