LMTV Park Brake not working

[DSD277]

WHY ???? back off the adjusters, that WILL make things WORSE. if anything TAKE up the adjusters.

Backing off is what I think someone did in the first place. Notice the "ed" for past tense in the post .

 

[JR1081]

Hey sorry to hijack this thread but mine just did the same thing From one day to the next. So not caged but rolls with parking brake pulled

 

[Ronmar]

I'm stumped sorry. This I don't understand: "Also park brakes don't apply with lack of air pressure". What on earth is holding the spring brakes back? Not a caging bolt, we see that. How could ALL 4 rear cans have frozen wedges? That is the only plausible explanation. Have someone actuate the brakes from the cab while you watch to see if the shoes move out to the drum on the rear. If the shoes do not move, you have front brakes only and your rear brakes need to be disassembled and freed up.

Hey sorry to hijack this thread but mine just did the same thing From one day to the next. So not caged but rolls with parking brake pulled

it just started? Is your front emer/red gladhand capped or frozen? The fronts must be vented. If they can trap air it will keep the park air applied to the spring chambers after you release park air at the control in the cab. If the blue service gets capped, it can cause the service brakes to stay on after you release the pedal…

 

[JR1081]

it just started? Is your front emer/red gladhand capped or frozen? The fronts must be vented. If they can trap air it will keep the park air applied to the spring chambers after you release park air at the control in the cab. If the blue service gets capped, it can cause the service brakes to stay on after you release the pedal…

That did it. Thank you!!!

 

[coachgeo]

..... The fronts must be vented. If they can trap air it will keep the park air applied to the spring chambers after you release park air at the control in the cab. If the blue service gets capped, it can cause the service brakes to stay on after you release the pedal…

clarifying- out in the real world trucks reaction

> if Red gets frozen/blocked back tires lock up if was previously in park. ...

??? what if was not in park first (froze via wet freezing conditions while driving?) Would a back set of brakes slowly get applied until they lock and front is unaffected?



> if blue Service...... gets frozen/blocked while sitting long cold period; all tires would be locked up and being previously in park would be irrelevant correct? Draining the tanks would release these...... correct? Granted you still have to unfreeze the front blue line...... or they would just slowly lock up again as pressure builds back up when compressor re-fills the tanks??

??? what if was not in park first (froze via wet freezing conditions while driving?? If I understand right... all the service brakes would begin to slowly apply then eventually lock up?? sound right?

thanks in advance.

 

[Ronmar]

When a front gladhand freezes it makes that line act like an air storage tank on one leg of a 2way check valve.

In the case of the park brakes, you apply air to one leg of a 2way. It passes thru that to compress the park springs and RELEASE the brakes. If the other 2way leg isnt vented, air can leak past and charge that "storage tank" to park air pressure. When you release park air intending to set the brakes, the stored air in the gladhand line will shift the 2waycore and continue to supply park air and keep the park springs compressed untill it leaks away. Frozen red means You wont be able to SET the park brake.

The same thing happens with the blue side when you step on the pedal, service air from the treaddle leaks across the 2way and charges the other side with its frozen vent. When you lift off of the pedal, the stored air shifts the 2way core and continues to apply service air untill.that stored air leaks away. Frozen blue means You step on the brakes to stop, but when you release the pedal the brakes DONT release.

If you vent the tanks, the rear service brakes will release as that air is applied via a relay, but the park brakes will also probably set so it will be hard to tell what is happening. The air will still be applied to front and to the control port on the rear relay from the independent air tank created by the frozen gladhand, untill it leaks away. You can vent this air by loosening one of the rear fittings on the lower pair of 2ways under the drivers step. It is the side where both valves are tied together into one line. Just leave it loose, that is the same as a vented front gladhand...