M1078A0 Parking Brake Diagnostics

[Lugnuts]

Well, by the title it sounds easy!!!
By the way, a salute to RonMar on an incredible explanation of the air system in these trucks. Thank you Sir.

I have a set of parking brakes that release anytime there is air in the system.
Now I can release them if I remove the line on the Inversion Valve, the one that feeds the Anti-Compounding Valve aka Parking Brake Valve.
I cut to the skinny on diagnostics.
So now having diagnosed the fact that everything is working on the rear correctly and the reason they are releasing is because I have air pressure coming out of the line that goes to the Parking Brake Release Valve on the dash.
The schematic in the manual shows that it goes from the dash valve to the Inversion valve.
So I remove the lines going to the Parking Brake Release Valve so I can locate my line going to the rear. I have air being blown from the rear, through the line so I can locate in the dash.
Having done that I start the truck to locate Supply Side Air and find that my Brake Release Valve is Plumbed correctly.
Next I reinstall Supply lines to Park Release Valve and start truck and low and behold I have air coming out the line that goes from the Park Release Valve to the Inversion Valve for the signal.
So I commence to check my Trailer Valve on the dash, start truck and it is not passing air to anything and another way to say it, it is working correctly but I still have air coming ut the line for the signal for the Inversion Valve.
Before I pull lines on the left frame rail, I figured I would write and ask for assistance. My thematic shows no relay valve in-between but by all indications there must be because I have air coming out that line and by the way it is not coming out on the other end of that line at the dash. The air is only traveling one way.

Any comments are welcome, (even derogatory) I thrive on those! Thanks guys!

 

[Ronmar]

From the park control in the dash, a line runs down to the floor forward of the brake pedal/steering column.
If you remove the grill you will see that it runs over to one input port of the top 2way check valve up behind the drivers step. the other input is from the front red glad-hand. The line from the red glad-hand Ts and also feeds a check valve that sends glad-hand air to the wet tank to charge the air system When being towed.

the output from the middle port on the 2way Ts and feeds a pressure sw that controls the park/emer lights in the dash and also feeds a remote control valve that controls air to the hydraulic pump, no hydraulics when park air is applied.

one thing that could be happening is that if that glad-hand was capped/sealed, and that check valve leaked, it would feed wet tank air thru the checkvalve, thru the 2way and back to the inversion valve to release the park brakes.

Since the main park brake control is from the two service tanks thru the park control, the quick check for this is to fill all the air tanks(dryer purges), then shutoff the engine. Park brake control pulled out/brake set, if the brakes are not set, then drain the wet tank. If the brakes set, then the source of the air is the wet tank and that can only be via the top 2way checkvalve.

Now of course the front glad-hands should be vented, and if that regular check was leaking wet tank air back, it should be pouring out that front red glad-hand instead of overriding your park control…

 

[Lugnuts]

WOW! First, I'm going to take a look at the schematics and see that! No, actually I'm first going to thank you for taking the time to answer the question and the smart's and experience you have behind that one!!! Secondly, I will look at that as soon as I can get back to it.
I know that the brakes set when the air is exhausted from the tank. That is the only way that they will set. Otherwise, any air in the system and they are released. I pulled lines from the Trailer Valve on the dash and did not have air at any of those ports but I never thought of removing covers from the Glad Hands themselves.
The other thing you stated was that there ws no air to the hydraulic pump system when the air brakes were released. Not that I don't believe you but I never had cause to try it. I will prove that because you never know when that may be a useful piece of information. Thank you very much!

 

[MatthewWBailey]

The other thing you stated was that there ws no air to the hydraulic pump system when the air brakes were released.

He means no air to the hydr pump when "park air" is applied, which releases the brakes. Hydr functions can't work when truck is moving, safety feature.