Parking brake not releasing

[Hanc]

Hello All
First time posting a question. After owning fixing a deuce (Lot of knowledge gained from this site )decided to buy a LMTV M1078 its 1995 built unit. Had visual issues but started and drove fine. Bought it, got it delivered drove it into the front yard and a day later tried to move the unit to the shed which is on top of a hill truck will not move. Started troubleshooting all the visual issues are coming back to bite me park brake will not release.I'm aware of the common issue with LMTV with check valves and its on order. The previous owner blocked the vent port on the brake valve not sure why and I have a mysterious hose with a valve which I cannot trace back leaking air all time and leaks a lot of air when brakes are released possible connected to something else to vent its soft to the touch and metal wire wrap around it. My question is can any of you can identify and tell me where its going or coming from. I'm reading the TM's it will take me some time before I find it. Any help is appreciated. Thank you in advance. I have attache pics for reference.

 

[Ronmar]

Allrightythen…

The pic with the short hose and pipe fitting and cap is your treadle valve. That capped line is connected to the valve vent. The treadle valve gets primary and secondary air from the tanks, and when you step on the pedal sends regulated air(more foot, higher pressure) to the primary(rear) and secondary(front) brakes to apply service brakes. When you let off the pedal, the treadle valve vents air thru that vent to lower the output pressure and ease/release the brakes. Capped like that your service brakes will never release… remove the cap. It should only vent air when you release the pedal. If it is venting continuously, remove the treadle valve clean and inspect it inside and either overhaul or replace it… sometimes the internal supply valve seat gets dirty and it allows supply in and this passes right out the vent with no pedal applied…

The last pic with the valve on the spiral line look like the vent hose. that line actually should be connected to the treadle valve vent. The park valve vent and the treadle valve vent are T’d together behind the heater and vent thru a floor port in front of the heater down in front of the radiator. If you remove the grill or tilt the cab you should be able to see it on the underside of the cab hump at the very front. the only time you should get air on that line where that valve is, is when you vent park air to set the park brakes, and then only until the park air vents away. A steady leak on that valve = a bad park control…

When standing beside the truck if you open the cab door just inside the door at eye level is where the pri and secondary air enter the cab to the treadle. There is a 2way check valve there that samples both and sends whichever is greater over to the park control valve. Your air pressure sensors are also there.

over at the park control, when you push it in, it sends that pri/sec air from the door 2way, to the trailer valve to enable it, and down to a 2way check under the drivers floor, then back to the anti compound valve above the rear axle. The anti-compound valve is a relay and seeing this air on its input, applies air down to compress the park springs and release the brakes. The 2way under the drivers floor(uppermost of 3) allows the red front glad-hand to also release the park brakes from a towing vehicle. There is also a pressure switch there that lights the park and emer lights in the dash…

When you pull out the park control, it cuts off the supply air and vents the air in the line down to the anti-compound valve out of that vent port behind the grill And turns on the park/emer lights in the dash. The anti-comp valve seeing this pressure drop on its input, copies it by releasing the spring air and the springs then re-apply park brake force setting the brakes.

i have done a couple vids on brakes over on youtube. Watch the brake overview and the front glad-hand delete vids. There are a few other air system/brake vids, I guess I should do a treadle valve video to complete the set

Here is the rear air brake component video, you should be able to find the others from there…

 

[Reworked LMTV]

OMG dangerous cobble job.. May have had leaking valve and decided to cap it off.

 

[Ronmar]

OMG dangerous cobble job.. May have had leaking valve and decided to cap it off.

Yea I really love people who drive an 8 ton vehicle and don’t understand the brakes…:O

 

[Hanc]

Ronmar thank you a bunch for the explanation. The guy who owned was 72 yrs old and he drove to work like that. I did take off the cap from vent of the threadle valve now the truck moves but I’m getting continuous air from the vent line will do the troubleshooting. Thank you all I have something now to start and look for and fix.

 

[Hanc]

OMG dangerous cobble job.. May have had leaking valve and decided to cap it off.

I removed the cap and the valve doesn’t leak. It vents when brake is applied and it’s loud.

 

[Ronmar]

I removed the cap and the valve doesn’t leak. It vents when brake is applied and it’s loud.

Yea when that spiral line is connected to the treadle valve vent all that park vent air should go out the floor port above the radiator…

 

[Hanc]

Yea when that spiral line is connected to the treadle valve vent all that park vent air should go out the floor port above the radiator…

I know I’m asking for more. What makes that hose vent all the time when engine is running or off and vent a lot when park brake released. I hear air from emergency glad hand venting too non stop.

 

[Ronmar]

The treadle/pedal valve vent and the park valve vent are tied together to the floor port. Air constantly coming out of them indicates a bad treadle or park air valve.

Air coming out of the red/emer glad-hand could be from 2 different sources. That gld-hand line splits and goes 2 ways. One is thru a check valve that feeds air to the wet tank to charge the towed vehicle air system when being towed so the vehicle brakes can be remote controlled by the front blue service glad-hand from the towing vehicle.

the second path is to that top 2way checkvalve under the drivers floor/behind the grill. This path allows the red glad-hand to release the park brakes. If the red glad-hand vents when the park brakes are released, the 2way is leaking. If it vents when the engine is running but park brakes set, that normal check is leaking back air from the wet tank. That check is located down in the bumper directly below the red glad-hand. I show it in the front glad-hand delete video…

 

[Hanc]

The treadle/pedal valve vent and the park valve vent are tied together to the floor port. Air constantly coming out of them indicates a bad treadle or park air valve.

Air coming out of the red/emer glad-hand could be from 2 different sources. That gld-hand line splits and goes 2 ways. One is thru a check valve that feeds air to the wet tank to charge the towed vehicle air system when being towed so the vehicle brakes can be remote controlled by the front blue service glad-hand from the towing vehicle.

the second path is to that top 2way checkvalve under the drivers floor/behind the grill. This path allows the red glad-hand to release the park brakes. If the red glad-hand vents when the park brakes are released, the 2way is leaking. If it vents when the engine is running but park brakes set, that normal check is leaking back air from the wet tank. That check is located down in the bumper directly below the red glad-hand. I show it in the front glad-hand delete video…

Thank you. I’ll report back when fixed.

 

[Hanc]

Thanks again Ron. Truck is up the hill in my shed I still have the leak when parking brake released will swap out parking brake valve and connect all hoses as it should be. Was nervous before I posted the question and I'm confident I can fix the issue. Thanks once again.