brake question for new m923 operator

[fireman1212224]

First off I love this forum. Now down to business. I am a full time heavy truck mechanic so I'm not totally stupid. The volunteer fire department where I am the head of maintenance as well as a fire fighter/emt. We have what I believe to be a m923. What does the spring brake override knob on the dash supposed to do? It does not set and release the brakes like normal semis do. If you lift the hand brake lever between the seats you hear a goose honking and the brakes will set. Pushing and pulling the knob on the dash makes the normal sounds like charging and releasing brakes would do but there is no drag at all with the knob out. Full air pressure on the gauges and the truck stops on a dime with the brake pedal. Can someone help?

 

[fireman1212224]

Also I dont know where to find the year but there is a tag on the engine that says 5/83

 

[gimpyrobb]

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(and twerking)

 

[MtnSnow]

The spring brake override can be used when you use an external air source to charge (release) the brakes (push in on the knob once there is enough air pressure to release the brakes and it will hold the brakes released even when you uncouple the external air source to the "red" gladhand or right side of the truck. this would allow you to push/pull the truck around but the second you hit the brake pedal or pull the knob the brakes will be reapplied until you hook up the air source again. (basically this allows you to move the truck without having to "cage" the brakes. (I actually did this to get my truck off my haulers trailer as GL or someone playing with the truck left the battery switch on and the batteries were to flat to start the truck when it arrived home)

You most likely have a '83 model truck

 

[fireman1212224]

So I should only use the hand brake to "park" the truck then? That works for me. I just wanted to be sure I didn't have an issue. Thank you

 

[doghead]

There trucks do not have the standard air brake(park and trailer supply) that civilian trucks have.

We have all the technical manuals here on the site. I suggest you download all of them for the M939 series trucks.

Read the one that ends with -10, this is the basic operators manual. Nobody should drive that truck without reading that one manual.

 

[RustyM923]

So I should only use the hand brake to "park" the truck then? That works for me. I just wanted to be sure I didn't have an issue. Thank you

Yes, the handle (when up) will release the air to the spring brakes and also set the parking brake. By twisting the top of the parking brake handle, you can loosen or tighten the parking brake. You'd need to hit the spring brake override (IN and with air pressure in the tanks) to check its effectiveness, however.

 

[fireman1212224]

I need to see if I can't adjust the parking brake shoes or band or whatever it has on the transfer case. I have tightened the knob all the way on the hand brake and you can feel a tiny bit of resistance but not like it should.

 

[RustyM923]

I need to see if I can't adjust the parking brake shoes or band or whatever it has on the transfer case. I have tightened the knob all the way on the hand brake and you can feel a tiny bit of resistance but not like it should.

Make sure you're adjusting it when the brake is off. (lever down)

 

[fireman1212224]

Oh yes. I have adjusted those before on older Spicer transmissions.

 

[73m819]

I would not be to worried about the band brake because the spring brakes SET when the handle is pulled up, if I had a 939 series, I would do away with the e brake lever all together and put a standard control valve in the dash, if the 939 series was a tractor, I would install the trailer control valve as well.

 

[FullSpecial]

Having a CDL since it was called a Combination License, that lever for a air parking brake defies order in the universe like
an open bag of Cheetos. I may get around to changing that out myself.

 

[wheelspinner]

The spring brake over ride's "prime" function is an emergency air supply to quickly move the truck from immediate danger after a complete loss of air. it uses a discreet tank. For instance you lose air and stopped in an intersection, railroad tracks, fire fight, etc....hit that button in, and you get to move until you apply the brakes again. Also, if the knob is pushed in, you will be parking only on the drive shaft manual brake.

 

[Floridianson]

The TM's say to air up the system and push in the over ride button but leave the handel up or brake on transfer case drum brake. Hold the foot brake and put the transmission in 5th gear and let up on the foot brake. If the drum brake does not hold by adjusting slack adjuster then you have to check what is wrong inside drum brake. Plus what was said above.

 

[rangereter]

With respect to to parking brake comment, by the time that the air pressure in the spring-brake chambers vents/dumps when the parking brake is initially applied, my truck can roll quite a distance (10 feet or more) if on a slope...I also realize that the service brake should be kept "applied" during this long drawn out process. One good thing about a "properly" adjusted mechanical parking brake is that it will instantly brake/arrest the drive-line and hold the truck in place (before you can roll into your wife's car in the driveway or something like that). I think some QRV,s (quick release valves) located closer to the drives would definitely speed-up the P-brake applying time.
Regards, Bob