M1078 Air brake help

When I apply the brakes, they all engage. However, I have a soft pedal, air releases out of the bleeder valve on the tractor protect valve, and I lose air pressure fast. I did have to replace both check valves above the drivers front tire. So, I thought maybe I had crossed a line or something. But after double checking, the connections appear good. I did use replacement DC-4 two way valves rather than the actual high pressure valves, but I'm not sure why/if that would matter. Any thoughts?

Thanks

Your tractor protection valve should not come into play unless you have the trailer brakes released or the trailer brake button on the dash pushed in. If memory serves me correct. If you are connected to a trailer and releasing its brakes then you could have a leak on the park cylinder inside a brake chamber leaking back up the service line.

Thanks for the feedback. No trailer. Could the same thing be happening with one of the rear vehicle chambers. They were both caged when I got the truck and who knows how long it sat like that. Although, they do engage. Maybe I have a problem with dash valve. I'll check that next. I guess it could also be the protect valve or even the bleeder, but I've been told they very rarely fail. I just feel that since some of the system was disassembled when I got the truck, that its something related to that. Of course, you never know. That's part of the "fun".

Most likely they had the brake chambers caged because it made it easier to move unit around the yard without building air. You can check the chambers by taking the service brake line off the chamber and releasing the brakes and see if air comes out the fitting on the chamber which it should not. Make sure to chock the wheels before you release the brakes. It's been a while since I worked on this kind of problem. But the tractor protection valve I don't think comes into play until you push in the trailer brake dash valve. So it is possible that the lines could be on wrong or the dash valve is bad. Yes the fun is tracking down the problem!

This is my first air brake truck. So, please excuse my basic questions. Should air be present at the rear glad hands by simply engaging the supply valve or do the brakes need to be applied to get air flow?

GTR0419 said:

This is my first air brake truck. So, please excuse my basic questions. Should air be present at the rear glad hands by simply engaging the supply valve or do the brakes need to be applied to get air flow?

Click to expand...

The emergency gladhand will only be charged when the trailer brake valve is engaged. The service gladhand will be charged when the foot brake is applied along with the trailer brake valve engaged.

NDT said:

The emergency gladhand will only be charged when the trailer brake valve is engaged. The service gladhand will be charged when the foot brake is applied along with the trailer brake valve engaged.

Click to expand...

Ok. So as far as the service side goes, that's what we have. However, if you are you saying that with the trailer valve engaged air should flow freely from the emergency side glad hand, that we do not have. Is that what you are saying? If so, maybe the trailer valve is indeed causing our problem.

GTR0419 said:

Ok. So as far as the service side goes, that's what we have. However, if you are you saying that with the trailer valve engaged air should flow freely from the emergency side glad hand, that we do not have. Is that what you are saying? If so, maybe the trailer valve is indeed causing our problem.

Click to expand...

I can triple check tomorrow, but I routinely use the trailer emergency gladhand to supply air to my 1" air impact, by pressing in the trailer brake valve.

Yes you should have air at the park/ emergency glad hand if you release the trailer brake dash valve if you do not have air there you have some kind of problem.

Based on what you two have told me, there must be something going on with the trailer supply valve. I am able to get air free flowing on the emergency side, but only with the parking brake off, which may indeed be normal. However, with the parking brake off, the trailer supply engaged, and my wife pumping the pedal, there was never any air flow from the service side.

I would replace the dashboard valves to begin with. One is on ebay right now. They are commercial items.

Yeah, I saw that one the other day. I hated too, but bought it. It's not fair on my part, but I always feel like I should get a deal on eBay; not pay retail or in this case more (Peach State Trucks quoted me $30, but would have to order it). I went ahead and did it because I should have it Tues/Wed, won't have to go pick it up, and I know it's the right one. Now I need to go find the service valve.

I would go ahead and check the service or TM manual and make sure all your hoses on the brake valve are hooked up correctly. One hose in the wrong spot could cause your problems or at least make your air act weird.

Good idea. You never know what's taken place before these things get into your hands. I knew a few check valves had been removed and hoped simply replacing them would give me a working system. I now think those checks were removed to use on another vehicle because there was a different system problem on this truck. Ugh. However, this is a good way to learn about airbrake systems. I will say that the TM pictures were not accurate for the check valve connections in the drivers wheel well. When I first attacked this problem I went to the GL yard and took some pictures from multiple vehicles to make sure the lines were correct. Those pictures varied from the TM.

So, it does not appear to be either of the dash valves. I switched from the Brake TM troubleshooting to the Air System TM troubleshooting and it does appear to be the tractor protection valve. I was actually able to find one and will install it as soon as it arrives.

Success! It was indeed the tractor protect valve. Installed new one today. No leak. Pressure holds. Brakes good.

Frustrating and bought a part I didn't need, but a good learning experience. One thing of importance that I learned is that some of the air brake components can be replaced with other part #'s. Of course that doesn't mean you should just slap anything on your rig and hope it works, but if you have a good parts guy at a truck parts store, they can find you a part to do the job. You may have to re-route lines to different ports, but if you have the original and know how it is set up you can make it happen. I always like to use an OEM part if possible, but you can't always find them.