Park Brake Won't Apply

[wrench31]

I'm troubleshooting an air park brake system on a M1083 FMTV. I have replaced the Dash park brake valve and the park control two way check valve under the dash on driver side. Still no park brake. the manual only states if releasing of air can be heard. other than this basic troubleshooting is there anything more in depth before I order an inversion valve and hope it works.

thanks.

p.s. system does build air pressure and no leaks can be heard/found.

 

[Suprman]

Take both front gladhand covers off see if air is coming out of the front red one.

 

[wrench31]

did that and no air was coming out

 

[Suprman]

If you drain all the air from the tanks with the truck off will it still roll?

 

[wrench31]

when the air is drained from tanks, the park brake does engage. with truck running I can release the park brake but it won't apply and it doesn't make the usual discharge sound

 

[Suprman]

I have seen trucks with a swapped air line. With the parking brake not working can you step on the brake pedal and stop the truck?

 

[tennmogger]

Keep in mind that no air is needed to set the parking brakes. If all the air leaks out anywhere, the brakes should set. So do you hear the air dump when you pull the yellow knob? If the air dumps and brakes still do not engage then there's a mechanical problem (the wedges are not being applied to the shoes to spread the shoes).

Are the caging bolts in place, disabling the parking brakes?

If brakes are not caged, do both parking brakes (one each end of the axle) fail to apply, or just one? If both stay non-applied then that would indicate either both have mechanical binding (not likely since they are totally independent mechanically) or that both still have air applied.

If one parking brake applies but one does not then air did get dumped and there is a mechanical problem with the non-applied brake. BTW, if totally out of adjustment, that will disable the brakes too because the wedges can't reach the shoe activators.

[with new posts while I was typing, some of my comments no longer apply]

 

[NDT]

I think someone posted another cause of this is the check valve behind the front bumper being stuck.

 

[wrench31]

I can step on brake pedal and stop the truck.
the only place I hear any air dumping is out of the park brake air valve (yellow knob) none of the chambers are caged and brakes shoes are adjusted to engage

 

[wrench31]

the valve behind the front bumper cause not hearing any air dumped at inversion valve?

 

[Suprman]

There is a spring brake release air circuit off the red front gladhand. Meant to release brakes for towing. Usually what happens is the check valve behind the bumper is bad, and all the air will leak out the red gladhand. So to build air someone will find a way to block the airflow at or around the gladhand. Which will then inadvertently feed back into the towing release system and no parking brakes. The tm does have some troubleshooting flow charts for your brake issue. Is this something that just started and it was fine before or did you get the truck like this? I have had issues with anti compounding valve before but the symptoms were different.

 

[wrench31]

it was fine before when I first pulled truck in. i couldn't hear any air coming out of the front glad hand, but when i removed the dummy coupling, there was a big dump of air. the valve on the dummy coupling was clogged/coupling not seated properly. i know that it not venting on the service side would keep service brakes applied but didn't think it would be the same for the emergency side.
i appreciate all the assistance with this issue

 

[Suprman]

Take all your gladhand covers off and leave them off. Check for any blockages inside the gladhands themselves. Sometimes mud wasps will build nests and block them up.

 

[wrench31]

will do

 

[Roach1319]

My friend checked my park brake issue and found its a bad Park Control Valve. Anyone have any ideas where I can find one?

 

[coachgeo]

My friend checked my park brake issue and found its a bad Park Control Valve. Anyone have any ideas where I can find one?

Your local Truck Supply, Napa etc. They are a standard commercial part. Check the sticky at top of this forum for the spreadsheet of part #'s.

 

[Roach1319]

Thank you for the info.
Guys I am fairly new here and I appreciate the help. I just got my M1078 and the the park brake was the only issue that needs to be addressed. Crossing my fingers that's all it will be.

 

[Roach1319]

Update the park brake is working as it should and I have put over 150 Miles on the truck this week alone.

 

[TC Fleet]

I ran into no park brake issue, checked the dash supply valve- working, all relay valves for trash- none and the inversion valve. Found air was always being supplied to the inversion valve no matter the position of the dash valve was in. after tracing all the air lines I found a check valve under the front bumper right behind front glad hand was not working, I had plugged the lines and parking brake worked. Replaced the check valve and everything is working fine. Hope this helps.