My cockpit contribution, I had to drive this truck one day while mine was in the shop.

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Beau_brooklyn@reddit

Thanks for this. I was just telling my students about the “blue valve” last week.

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It releases the tractor brakes , but you have to hold it down (like the treadle brake for the trailer) and it has its own air tank. So, it’s used if the tractor needs to be moved, like in emergency situations, if you don’t have any air.

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SawyerJWRBLX@reddit

Had to? More like got to

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MoosiePie22@reddit

Probably going to show my age here but what is the blue one 😅

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r_s-nsfw@reddit

Where I work, the blue valve applies the service brakes when only the trailer brake is released. It's a way to see if the trailer brake lights are working correctly & check all air connections. I pull doubles, so it's useful to make sure there are no leaks in airlines on the dolly.

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odinskriver39@reddit

That's how we used to pre-trip sets of doubles. Safer than holding the handbrake with the seatbelt.

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trakr24@reddit

Yellow - Tractor Parking Red - Trailer Air supply Blue - Trailer parking

You can use the Blue valve to engage the Trailer brakes without draining the Trailers air tanks. Wish modern trucks had it. All out Fontaine flatbeds dump their air tanks when you pull the supply and it deflates the Trailer Air bags. Causes brakes to slip from the weight of the Trailer settling. Have the release the Tractor Parking brakes to release the tension.

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CashWideCock@reddit (OP)

I don’t think that’s accurate, I used it as a parking brake even when bobtail. As a matter of fact, the truck in my pictures is a straight truck flatbed and the truck would not move until I pushed both the yellow and blue buttons.

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trakr24@reddit

Huh, maybe your truck is set up different. I was taught that the blue plunger was a parking brake for trailers when spring brakes werent present and just had air brakes. Was more common back in the day when trailers didnt have spring brakes as standard.

Maybe your truck has an air interlock that makes you have to press both to completely disengage the parking brakes on the truck. Probably like that so you dont burn up the brakes on the trailer.

Also your truck might have been a Tractor at one point and was converted to a flatbed straight truck.

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CashWideCock@reddit (OP)

I really don’t know the right answer, but I drove 72 , 74 , and 86 year models of Kenworths that had the blue button. All 3 of them required both the yellow and blue buttons to be pushed in to drive, even without trailers.

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1morepl8@reddit

I hate auto air trailers so much. That's all.

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icy_penguins@reddit

From what ive read, the blue valve was to release the tractor brakes if you lost air and your brakes set. It had its own separate air tank that could get you out of the way. I always thought it was for trailers before they had spring brakes on them. Push the blue one to set the trailer brakes so you could hook up to it. I remember that trailer dollies used to have wheels.

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CashWideCock@reddit (OP)

I’ve driven 3 different trucks with the blue valve and I’ve never known the exact purpose. I used it as a parking brake, same as we use the yellow on today. You’ll get lots of different answers from other drivers about the purpose of it.

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amazingmaple@reddit

Nice old mack.

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CashWideCock@reddit (OP)

Not a Mack.

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amazingmaple@reddit

Hahaha. Yup. I didn't pay attention to the steering wheel. The right dash looks very similar to an r model.

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imcjoey13@reddit

And the gps is where?!?

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CashWideCock@reddit (OP)

On my phone.

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CashWideCock@reddit (OP)

I cut my teeth on old iron, I appreciate old iron, old iron is cool, I just don’t want to earn my paycheck with old iron.