M1078 Drives Through Brakes

[AmbitionStrikes]

The truck: 1994 m1078

Condition: Has sat for several years, owner said the last time he drove it the brakes worked fine. Located in CA, temps are well above freezing.


Issue: When in gear the truck drives through the brakes, even at idle. If you engage the parking brake, it locks up and stops immediately. The brake pedal goes down to the floor with medium resistance. The air pressure gets up to 90+ psi and we can’t hear any leaks when holding the pedal (with the truck off so we can hear). Air pressure will hold overnight.


Diagnosis: Air was rushing out of the front emergency brake gladhandle. We took apart the check valve behind the bumper, cleaned it up (it was full of gunk) and upon reassembly no more air coming out.


We downloaded the TM and followed the procedures in i2: Rear Brakes Do Not Apply. We couldn’t find any issues and it seems that air is being delivered through the system and to each wheel. We can visibly watch all 4 brakes being applied to the wheel when we press the pedal.


We didn’t have the correct tools to properly check the shoe spacing. But it did not appear excessive (also the owner assured us they worked last time it was driven). The diagnostics for Extended Braking Distance allude to issues with the brakes on a specific wheel, not the system as a whole so we are hesitant to start pulling drums off in the guys driveway.


Our latest guess is that maybe not enough air pressure is being delivered to the brakes?


It’s our first vehicle with air brakes and we’re trying to learn as much as possible. Any input is greatly appreciated!

 

[Coffey1]

Mine would not stop it was a check valve at driver door step

 

[Andyrv6av8r]

Tractor protection valve diaphragm maybe.

 

[AmbitionStrikes]

Tractor protection valve diaphragm maybe.

Wouldn't this manifest as a leak? We can't detect any leaks.

Another piece of info: If the park brake is set (air released) and I step on the service brakes with the truck in gear it will start moving until I release the service brakes.

 

[Andyrv6av8r]

Wouldn't this manifest as a leak? We can't detect any leaks.

Another piece of info: If the park brake is set (air released) and I step on the service brakes with the truck in gear it will start moving until I release the service brakes.

So parking brakes hold but release when you push the brake pedal? Sounds like something is plumbed wrong. If you put air on the front glad hand with the parking brakes on ( like getting ready to tow), the park brakes release. So look into that circuit.

 

[AmbitionStrikes]

So parking brakes hold but release when you push the brake pedal? Sounds like something is plumbed wrong. If you put air on the front glad hand with the parking brakes on ( like getting ready to tow), the park brakes release. So look into that circuit.

The truck was driven regularly by the previous owner until parked 4 years ago and the brakes worked fine. They haven't touched it since parking it. Although what your saying does make me start thinking about stuck check valves.

Is there a good diagram or schematic showing all of the air plumbing?

 

[Andyrv6av8r]

Just for the heck of it can you hook shop air up to the glad hand to check that the parking brakes release?

 

[AmbitionStrikes]

Just for the heck of it can you hook shop air up to the glad hand to check that the parking brakes release?

I'll add that to the list of things to check today. The truck is parked at the previous owners home an hour away.

 

[Ronmar]

Last symptom has to do with the anti-compounding valve and is perfectly normal. The rear wedge mechanisms can be damaged if both park spring force AND service brake force are applied together. If no park air is applied to release the springs, the anti-compounding valve applies it when service air is applied, to remove the spring force. All trucks do this, but we don’t know it because the service air applied to all 4 wheels should override the spring force removed from two wheels.

your problem is low air pressure at the cans, or brakes mis-adjusted(or both). I tend to go with low pressure as the park brake appears to be working correctly.

The pedal/treadle valve is a lever controlled pressure regulator. Harder you press = greater PSI on the output. +PSI = +braking force. When you step on the pedal, air is fed thru some 2way check valves under the drivers floor and to their front QR or rear relay valves. In the front the QR is a remote pressure regulator Whatever pressure is applied to the input, it mimics on the outlet. The outlets run to the actuators. When you release it vents the actuator air down there so it can release quickly to help control brake lock.

the rear is a little different, it flows thru a load proportioning valve(lever actuated regulator like the treadle) to a relay valve. A relay is also a remote pressure regulator, and maintains output based on input, but it does so by using a supply direct from the primary air tank. You have 4 actuators back there and if all their air had to come from the treaddle it would take a while to flow enough air to build PSI and braking force... the relay can deliver air volume faster to the 4 cans.

you need a pressure gauge with two different fittings. One fitting is a 1/2” compression fitting like all the plastic brake lines are attached to so you can disconnect a line from a fitting and connect it to the gauge. The other fitting is the same as what is found on the brake cans, so you can disconnect a actuator supply line and connect it to a gauge. Check pressure at the input to the front QR and rear relay valves with pedal application. Then check it at the can inputs with pedal application.

The dash gauges sense pressure at the treadle inlet. Running, the governor/compressor should push them to 120 without problem. If they are not making 120, you have an issue(compressor/dryer issue, leaks or bad gauges). Does the air dryer purge(go woosh) occasionally? It should purge every time the compressor reaches set pressure and unloads. no whooshes, the compressor isn’t making set pressure of 125...

it is hard to hear leaks with the cat purring so apply shop air to the front emer glad hand(this releases the park brake so chock the wheels also) and pressurize to 125 and test the system operation and look for leaks again.

2 separate systems(front and rear) with the exact same symptom kinda points me toward the treadle valve not delivering high pressure. Especially if the brakes worked previously. But you need to check pressures to nail this down...

 

[Mullaney]

The truck was driven regularly by the previous owner until parked 4 years ago and the brakes worked fine. They haven't touched it since parking it. Although what your saying does make me start thinking about stuck check valves.

Is there a good diagram or schematic showing all of the air plumbing?

.
Sometimes the hardest thing to do is ignore what the previous owner tells you. MOST folks will tell you the facts as they know them.

On the other hand, when "nothing is adding up" sometimes ignoring what the previous owner told you and start with the basics - like applying air to the red gladhand on the front of the truck can't lie. It works or it doesn't...

Ronmar gave you a lot of information. The schematic is there in his post too. Simple is good. Block / Chock the wheels and try airing up the system from the front of the truck.

Would love to know the results of that.

 

[AmbitionStrikes]

Last symptom has to do with the anti-compounding valve and is perfectly normal. The rear wedge mechanisms can be damaged if both park spring force AND service brake force are applied together. If no park air is applied to release the springs, the anti-compounding valve applies it when service air is applied, to remove the spring force. All trucks do this, but we don’t know it because the service air applied to all 4 wheels should override the spring force removed from two wheels.

your problem is low air pressure at the cans, or brakes mis-adjusted(or both). I tend to go with low pressure as the park brake appears to be working correctly.

The pedal/treadle valve is a lever controlled pressure regulator. Harder you press = greater PSI on the output. +PSI = +braking force. When you step on the pedal, air is fed thru some 2way check valves under the drivers floor and to their front QR or rear relay valves. In the front the QR is a remote pressure regulator Whatever pressure is applied to the input, it mimics on the outlet. The outlets run to the actuators. When you release it vents the actuator air down there so it can release quickly to help control brake lock.

the rear is a little different, it flows thru a load proportioning valve(lever actuated regulator like the treadle) to a relay valve. A relay is also a remote pressure regulator, and maintains output based on input, but it does so by using a supply direct from the primary air tank. You have 4 actuators back there and if all their air had to come from the treaddle it would take a while to flow enough air to build PSI and braking force... the relay can deliver air volume faster to the 4 cans.

you need a pressure gauge with two different fittings. One fitting is a 1/2” compression fitting like all the plastic brake lines are attached to so you can disconnect a line from a fitting and connect it to the gauge. The other fitting is the same as what is found on the brake cans, so you can disconnect a actuator supply line and connect it to a gauge. Check pressure at the input to the front QR and rear relay valves with pedal application. Then check it at the can inputs with pedal application.

The dash gauges sense pressure at the treadle inlet. Running, the governor/compressor should push them to 120 without problem. If they are not making 120, you have an issue(compressor/dryer issue, leaks or bad gauges). Does the air dryer purge(go woosh) occasionally? It should purge every time the compressor reaches set pressure and unloads. no whooshes, the compressor isn’t making set pressure of 125...

it is hard to hear leaks with the cat purring so apply shop air to the front emer glad hand(this releases the park brake so chock the wheels also) and pressurize to 125 and test the system operation and look for leaks again.

2 separate systems(front and rear) with the exact same symptom kinda points me toward the treadle valve not delivering high pressure. Especially if the brakes worked previously. But you need to check pressures to nail this down...

View attachment 833442

Thank you very much for the detailed reply and the diagram.

 

[AmbitionStrikes]

We paid to have a heavy wrecker tow the truck back to our house. Now it's going to be significantly easier to work on and diagnose. I bought fittings to hook shop air up to the front emergency glad handle today and a pressure gauge and fittings to test the pressure in the various lines. I will update this thread soon with more info. Thanks for all the help so far!

 

[AmbitionStrikes]

Last symptom has to do with the anti-compounding valve and is perfectly normal. The rear wedge mechanisms can be damaged if both park spring force AND service brake force are applied together. If no park air is applied to release the springs, the anti-compounding valve applies it when service air is applied, to remove the spring force. All trucks do this, but we don’t know it because the service air applied to all 4 wheels should override the spring force removed from two wheels.

your problem is low air pressure at the cans, or brakes mis-adjusted(or both). I tend to go with low pressure as the park brake appears to be working correctly.

The pedal/treadle valve is a lever controlled pressure regulator. Harder you press = greater PSI on the output. +PSI = +braking force. When you step on the pedal, air is fed thru some 2way check valves under the drivers floor and to their front QR or rear relay valves. In the front the QR is a remote pressure regulator Whatever pressure is applied to the input, it mimics on the outlet. The outlets run to the actuators. When you release it vents the actuator air down there so it can release quickly to help control brake lock.

the rear is a little different, it flows thru a load proportioning valve(lever actuated regulator like the treadle) to a relay valve. A relay is also a remote pressure regulator, and maintains output based on input, but it does so by using a supply direct from the primary air tank. You have 4 actuators back there and if all their air had to come from the treaddle it would take a while to flow enough air to build PSI and braking force... the relay can deliver air volume faster to the 4 cans.

you need a pressure gauge with two different fittings. One fitting is a 1/2” compression fitting like all the plastic brake lines are attached to so you can disconnect a line from a fitting and connect it to the gauge. The other fitting is the same as what is found on the brake cans, so you can disconnect a actuator supply line and connect it to a gauge. Check pressure at the input to the front QR and rear relay valves with pedal application. Then check it at the can inputs with pedal application.

The dash gauges sense pressure at the treadle inlet. Running, the governor/compressor should push them to 120 without problem. If they are not making 120, you have an issue(compressor/dryer issue, leaks or bad gauges). Does the air dryer purge(go woosh) occasionally? It should purge every time the compressor reaches set pressure and unloads. no whooshes, the compressor isn’t making set pressure of 125...

it is hard to hear leaks with the cat purring so apply shop air to the front emer glad hand(this releases the park brake so chock the wheels also) and pressurize to 125 and test the system operation and look for leaks again.

2 separate systems(front and rear) with the exact same symptom kinda points me toward the treadle valve not delivering high pressure. Especially if the brakes worked previously. But you need to check pressures to nail this down...

Thank you so much for this detailed post.

We hooked up shop air of 100 psi to the front emergency glad handle.
Pressure at the output of the rear relay valve was 80psi. Unhooked the LSPV arm to simulate a load and then got 100psi.

Started the truck and let it get up to pressure (waited for the purge at the dryer)
Then when we hit the brakes we had 118 psi at the output of the rear relay valve.

Then we checked pressure at the output of the front quick change valve. 118psi.

So, from what I can tell that rules out the air delivery system as the problem.

It can either be:
1. the brake chambers not applying enough force (leaking?)
2. the shoes misadjusted (but the parking brake works great)

Does anyone have any other ideas or am I missing something?

If the brake chambers were leaking, wouldn't we hear a big leak?

To reiterate, the brakes are barely strong enough to slowly stop the truck from a slow roll while in drive. Any throttle easily overcomes the brakes.

 

[Ronmar]

You can see the shoes move outward toward the drum when you step on the pedal right? need a feeler gauge to check the gap between shoes and drum. You are looking for .020” between shoe and drum measured in the middle of each shoe arch.

it seems weird that the spring brakes would hold but the service would not in the rear, as they use the same wedges/actuators. Can you follow the lines from the rear relay and confirm they are attached to the service ports on the rear cans?

you’r in sand point, you have mason bees and or mud daubers there? The actuators receive service air on one side of the main diaphragm to push a rod down into the wedge to actuate the brakes. The other side of the diaphragm has a port and a hose that runs up to a vent port on the frame. If these ports got clogged the trapped air on the backside of the diaphragm would act like a air spring and would reduce brake force(i have tuned waste-gate actuators like this)

 

[Coffey1]

Again on my wrecker I had rear brakes.
No front brakes and that big beast would not stop
Looked at check valve on drivers side step and got fronts back and the fat ass girl stops now.

 

[AmbitionStrikes]

You can see the shoes move outward toward the drum when you step on the pedal right? need a feeler gauge to check the gap between shoes and drum. You are looking for .020” between shoe and drum measured in the middle of each shoe arch.

it seems weird that the spring brakes would hold but the service would not in the rear, as they use the same wedges/actuators. Can you follow the lines from the rear relay and confirm they are attached to the service ports on the rear cans?

you’r in sand point, you have mason bees and or mud daubers there? The actuators receive service air on one side of the main diaphragm to push a rod down into the wedge to actuate the brakes. The other side of the diaphragm has a port and a hose that runs up to a vent port on the frame. If these ports got clogged the trapped air on the backside of the diaphragm would act like a air spring and would reduce brake force(i have tuned waste-gate actuators like this)

Embarrassingly, I don't own a set of feeler gauges. However, I couldn't slip a piece of .030 welding wire between the shoes and the drums at the little scallop in the center of the arch. So it seems like they are in adjustment.

Out of frustration I decided to try and take the truck for a drive. The rear brakes work now!

We had a heavy wrecker tow it the 60 miles or so to our house. He had the emergency air and service air hooked up through the front glad handles during the drive. The front wheels were up in the air for the tow.

My theory is that the shoes were out of adjustment, or the relay valve was stuck and physically using the brakes freed something up.

We live on a dirt road and if I mash the pedal the rear brakes lock up and the truck skids to a halt. It doesn't feel like the front brakes are contributing much.

Should I adjust the LSPV to try and get the front brakes to do more? Or should I just keep driving the truck up and down the road and see if things continue to improve?

Thanks again for all the help! I'm looking forward to contributing more to this community. I think this has been a good exercise for us at any rate, we now have a much better understanding of the brake system on this truck.

@Coffey1 I think the pressure tests I have done rule out those check valves as a potential problem. I have full pressure at the output of the quickchange valve in the front.

 

[Ronmar]

The LSPV only effects rear pressure delivered to the cans. It won’t increase front pressure as the front and rear systems are completely separate.

Actually i would strip off the front drums and actuators and inspect the wedge mechanisms(And once the front’s are done, I would do the rears also) It is not difficult, just large/heavy. wedges can deliver great force, but not much movement so it won’t take much out of wack to keep them from pinning the shoes out against the drums.

When I brought mine home it had low brake force on the front and pulled to the right. Upon inspection I discovered 3 of the 4 front brake shoes were installed backwards(they are stamped actuator end, Mmmm motor pool maintenance)... When installed backwards the shape of the shoe damages the star adjuster wheels and keeps the auto adjusters from functioning. Once out of adjustment with too much gap on the shoe, the wedges can hyper extend and get damaged. The wedge pushes between two rollers that run in ramped channels on the ends of the actuator plungers. one of mine had pushed the rollers out of their track, which disabled the brake completely on that side(caused the pulling). It was also full of nasty dried-out grease...

I would do a full inspection instead of trusting it to “fix” itself... I believe Neilvvt over on youtube did a good brake strip/overhaul video...

 

[AmbitionStrikes]

The LSPV only effects rear pressure delivered to the cans. It won’t increase front pressure as the front and rear systems are completely separate.

Actually i would strip off the front drums and actuators and inspect the wedge mechanisms(And once the front’s are done, I would do the rears also) It is not difficult, just large/heavy. wedges can deliver great force, but not much movement so it won’t take much out of wack to keep them from pinning the shoes out against the drums.

When I brought mine home it had low brake force on the front and pulled to the right. Upon inspection I discovered 3 of the 4 front brake shoes were installed backwards(they are stamped actuator end, Mmmm motor pool maintenance)... When installed backwards the shape of the shoe damages the star adjuster wheels and keeps the auto adjusters from functioning. Once out of adjustment with too much gap on the shoe, the wedges can hyper extend and get damaged. The wedge pushes between two rollers that run in ramped channels on the ends of the actuator plungers. one of mine had pushed the rollers out of their track, which disabled the brake completely on that side(caused the pulling). It was also full of nasty dried-out grease...

I would do a full inspection instead of trusting it to “fix” itself... I believe Neilvvt over on youtube did a good brake strip/overhaul video...

I guess when I said "get the front brakes to do more" I really meant "make the rear brakes do less so the front brakes have a chance to get up to adequate pressure before the rears lock up"

Thanks for the info. I have watched the Neilvvt brake video. Doesn't look too bad, part of me wants to just buy all new parts and go to town on rebuilding the mechanical part of the brakes.

Thanks again for the help! I'll report back as I learn and discover more.

 

[Mullaney]

I guess when I said "get the front brakes to do more" I really meant "make the rear brakes do less so the front brakes have a chance to get up to adequate pressure before the rears lock up"

Thanks for the info. I have watched the Neilvvt brake video. Doesn't look too bad, part of me wants to just buy all new parts and go to town on rebuilding the mechanical part of the brakes.

Thanks again for the help! I'll report back as I learn and discover more.

.
Remember, those front brakes need to apply after the rears. With normal Slack Adjusters, the back is between a quarter and a half turn. Front adjusters is backed off between a full turn and a turn and a quarter. You want them to work, but on wet roads in particular you want the fronts to apply after the rears.

Greasing the zerks and cleaning the rollers and the rollers and cams might make a really big difference.

 

[Ronmar]

.
Remember, those front brakes need to apply after the rears. With normal Slack Adjusters, the back is between a quarter and a half turn. Front adjusters is backed off between a full turn and a turn and a quarter. You want them to work, but on wet roads in particular you want the fronts to apply after the rears.

Greasing the zerks and cleaning the rollers and the rollers and cams might make a really big difference.

Wedge brakes, no zerks or cams, just rollers between the wedges and plungers, and you need to disassemble them to get to those for fresh grease...