M1078 brakes

[Phil S .]

Have a M1078 that was bought and while driving brakes would lock up. The air pressure was 95 psi, adjusted to 115 psi.
The service brake would engage when the park brake was released.
Checked for leaking air into system and found that the vent behind the grille has escaping air, plugged with finger and park brake valve released.
Found that the service brake valve had air escaping from vent. Disconnected line to rear service brake and no leakage there from park brake chamber.
Removed each outlet line at floor board and found air coming from port for front brake due to gauge loosing pressure.
With park brake valve engaged was able to move truck. Replaced lower check valve and no change.
Inherited truck from another tech so... Looking at diagram and looks like problem could be in the valve "anti-compressing"
Help would be appreciated.

 

[Ronmar]

Adjust the air up some more. The governor should cut out with the wet tank pressure between 120-125PSI. You should use a gauge on the wet tank to do this.

What service brake valve had air escaping from vent? The service relay over the rear axle?

Air coming from the vent behind the center of the grill(above radiator) indicates that a Park/Emergency valves in the dash may be defective/leaking, unless it is air passing thru a blown brake can when the service brakes are applied. But any air that would do that should be vented at the relay/quick releasing valves...

to check for blown brake can diaphragms you need to remove the service lines at each can with park air applied, and look for air coming right out of the service port on the can. The plumbing for all 4 rear cans are tied together so park air leaking thru To the service side on any can would be exhausted at the service relay Vent on the rear crossmember...

here is a video I did a few weeks ago on the function of all those rear components.

 

[Phil S .]

Adjust the air up some more. The governor should cut out with the wet tank pressure between 120-125PSI. You should use a gauge on the wet tank to do this.

What service brake valve had air escaping from vent? The service relay over the rear axle?

Air coming from the vent behind the center of the grill(above radiator) indicates that a Park/Emergency valves in the dash may be defective/leaking, unless it is air passing thru a blown brake can when the service brakes are applied. But any air that would do that should be vented at the relay/quick releasing valves...

to check for blown brake can diaphragms you need to remove the service lines at each can with park air applied, and look for air coming right out of the service port on the can. The plumbing for all 4 rear cans are tied together so park air leaking thru To the service side on any can would be exhausted at the service relay Vent on the rear crossmember...

here is a video I did a few weeks ago on the function of all those rear components.

Have air leaking out from the bulkhead on the floor to the brake pedal valve connection.
Disconnected both service brake lines at the rear axle brake chambers and no air coming from the brake chambers.
The front brakes should not come on with the parking brake correct?
Disconnected the hose at the front axle service brake and have air coming from the hose when the park brake is released.
Sounds like that valve on the cross member could be the issue.

 

[Ronmar]

Have air leaking out from the bulkhead on the floor to the brake pedal valve connection.
Disconnected both service brake lines at the rear axle brake chambers and no air coming from the brake chambers.
The front brakes should not come on with the parking brake correct?
Disconnected the hose at the front axle service brake and have air coming from the hose when the park brake is released.
Sounds like that valve on the cross member could be the issue.

Ok, There is no way park air should be able to make it to the front cans. it sounds as if you have some hoses criss-crossed. With the cab tilted, you can step in between the drivers tire and the frame. on the inner fender/cab wall between drivers step and steering box there are three 2-way check valves. A 2way check has two possible inputs(whichever is greater) On either side of the valve and one outlet in the middle of the valve body. What these 3 valves are for is to allow either the cab pedal and park valve to control the brakes, or the front gladhands With air from a towing vehicle, to control the brakes.

The uppermost one Is for park air, It has the input from the yellow park valve in the cab and an input from the front emergency gladhand(Front and rear ports) and an output(Middle port) which feeds to the rear valves to compress the park springs and release the park brakes. It has a pressure switch on its output that turns off the park and emer lights in the dashboard annunciator panel When you apply park air from either source.

Below that are two Valves that are connected together at the rear port(from front service gladhand) The front ports are either primary or secondary service air coming down from the treadle/brake pedal valve. The middle ports on these two valves feed to the primary(rear) brake system, and the secondary(front) brake system.

I think the only way you could see park air coming out of one of the lower pair of 2way check valves is if a park air line is incorrectly connected to it.

I removed my front gladhands and all those valves so I can’t show you pics. But I did up some video showing Exactly how I did it for a friend. In the vid I identify a bunch of the specific port functions which might help you confirm that things are connected correctly. Here is a link to it.

You said you inherited someone else's mess so y0u can’t take anything for granted...

 

[Phil S .]

Have air leaking out from the bulkhead on the floor to the brake pedal valve connection.
Disconnected both service brake lines at the rear axle brake chambers and no air coming from the brake chambers.
The front brakes should not come on with the parking brake correct?
Disconnected the hose at the front axle service brake and have air coming from the hose when the park brake is released.
Sounds like that valve on the cross member could be the issue.

 

[Ronmar]

???

 

[spoon6]

I'm following this thread for brake issues as well. Nice video

 

[Phil S .]

Does anyone have a diagram not a schematic of the brake system, have crossed line or bad valve but having problems following hose routings.
Disconnected front brake hoses but do not know where the supply comes from to the bulkhead by the bumper.

 

[Ronmar]

Does anyone have a diagram not a schematic of the brake system, have crossed line or bad valve but having problems following hose routings.
Disconnected front brake hoses but do not know where the supply comes from to the bulkhead by the bumper.

I don’t think there is an overall system diagram/as built drawing . They have a bunch of Localized drawings in the repair manuals associated with the replacement of each specific component. And in the 24P manual(parts exploded views) but they don’t show an overall layout. The 24P is handy for locating specific components.

As much of a PITA as it is, it is still probably easier to hand over hand the lines coming and going from each specific component Shown on the air system schematic.

one trick I use to help speed this up is to look at the schematic, and identify one line between its respective ports on 2 components. I then Disconnect the line on that port and apply air to one end of the line, expecting it to flow out of the other end of the line you disconnected. If it does, great you just id’d a line, if it does not flow air, then you need to hand over hand That line...

 

[Ronmar]

I did come across a point to point table for all the airlines In the manual and page number circled in this picture. The following pages have Drawings and tables that list the hose name and its start fitting and end fitting With numbers to identify their location on the chassis.

 

[Lugnuts]

I did come across a point to point table for all the airlines In the manual and page number circled in this picture. The following pages have Drawings and tables that list the hose name and its start fitting and end fitting With numbers to identify their location on the chassis.

View attachment 815076

Thank you Coachgeo