m105 trailer brakes lock up

[Duece Man 35a2]

This is my first post, so here you go. I have a M105 trailer and I believe it is the newer style brake system. Air goes into the trailer from rear of the truck, both sides, the service and the emergency lines. Goes thru the filters and into the air brake relay valve (2 sides) which is mounted to the air tank with a 1" nipple. There is a third line coming from the relay valve and goes to a diaphragm attached the master cyl. From the master cyl. to the wheel cyl. it's hydraulic (dot 5). Everything from truck to trailer is hooked up, air and electric. One side of the trailer on jack stand, tire spins freely. Truck gland valves closed both sides, trailer air tank empty. Open the service side only (drivers side) and step on the brake pedal. Go back to the trailer and the wheel will not rotate. Crawl under the trailer and empty the air tank/drain valve and the wheel spins again, air tank pressure let off. Close service side and open emergency side gland, wheel locks up. Air tank full again and must drain air again to release the wheel. Air relay valve seems to be doing nothing. Old valve and a new replacement are doing the same thing. The truck shop matched it up and they look the same. Could they still be different? What is the right part number? Going nuts!!!

 

[MWMULES]

Have you tried switching your air lines, one I got years ago was hooked up backwards and marked/painted wrong,

 

[Scrounger]

There could be several things going on.

First a little background on trailers. On newer all air systems, after 1976, when the emergency air line is charged the air tank(s) are filled and the spring brakes are released. When the brakes are applied, air goes through the service line, through the relay valve to operate the brakes. The trailer brake operating air comes from the trailer are tank(s). There has to be enough air in the system to release the brakes. If the air pressure drops, trailer disconnects, line fails, the brakes apply automatically to stop the trailer.

The M105 trailer use an air over hydraulic system. The newer trailers have emergency and service side systems. The emergency side charges the air tank and provide operating air. The service side provides the signal air for the relay valve. That is the same as full air systems. The difference is when the air level drops. When the relay valve has low pressure, it takes air from the tank and applies the brakes. Once the air pressure is raised the brakes will release. Or if the air tank is drained, the brakes will release because there isn’t any air to apply the brakes.

It doesn’t hurt to check the vehicle air system first. I use a glad hand with an air fitting, air line and a pressure gauge. Connect the air line to the service valve and run it to the cab and watch the valve when you apply the brakes. The pressure should go up when applying the brakes, but it must go to zero when released. If it doesn’t go to zero you have a problem. To check the emergency side, connect the air line to the emergency side and make sure the pressure is about the same as the cab gauge.

The trailer must have the emergency air side at full operating pressure to check the brakes.

If I were testing the trailer. I would hook the trailer to the vehicle, then place it on two jack stands.

With no air lines connected and the air tank drained I would check for free turning wheels, both sides.

Then connect the air lines and fully charge the system. Then check for free turning wheels. If the wheels don’t turn freely the relay valve is bad.

To check the trailer service side brakes, have soldier B apply the brakes and see if the trailer brakes come on. When soldier B releases the brakes the trailer brakes should release. If they don’t release or drag the relay valve needs to be checked. It may not be venting.

 

[Duece Man 35a2]

Have you tried switching your air lines, one I got years ago was hooked up backwards and marked/painted wrong,

Well, funny you should say that. I just laid under the trailer for about an hour THINKING and making sure that I had hooked up the new air relay the same as the one I took out, no mistakes. Then I went into the shop and looked at the one I removed, it's in real good shape. I'm standing there reading the writing by the inlets. I turn it over to look at the other side and see the words "service" and "emergency" by the 2 open inlets. If that side had been facing down I would have seen that. ALSO, that turns the valve 180 degrees and the air lines ARE crossed. You are correct!!! It was hooked up backwards from the beginning. I will know for sure tomorrow, I hope! I'll let you know. Many thanks!

 

[Duece Man 35a2]

There could be several things going on.

First a little background on trailers. On newer all air systems, after 1976, when the emergency air line is charged the air tank(s) are filled and the spring brakes are released. When the brakes are applied, air goes through the service line, through the relay valve to operate the brakes. The trailer brake operating air comes from the trailer are tank(s). There has to be enough air in the system to release the brakes. If the air pressure drops, trailer disconnects, line fails, the brakes apply automatically to stop the trailer.

The M105 trailer use an air over hydraulic system. The newer trailers have emergency and service side systems. The emergency side charges the air tank and provide operating air. The service side provides the signal air for the relay valve. That is the same as full air systems. The difference is when the air level drops. When the relay valve has low pressure, it takes air from the tank and applies the brakes. Once the air pressure is raised the brakes will release. Or if the air tank is drained, the brakes will release because there isn’t any air to apply the brakes.

It doesn’t hurt to check the vehicle air system first. I use a glad hand with an air fitting, air line and a pressure gauge. Connect the air line to the service valve and run it to the cab and watch the valve when you apply the brakes. The pressure should go up when applying the brakes, but it must go to zero when released. If it doesn’t go to zero you have a problem. To check the emergency side, connect the air line to the emergency side and make sure the pressure is about the same as the cab gauge.

The trailer must have the emergency air side at full operating pressure to check the brakes.

If I were testing the trailer. I would hook the trailer to the vehicle, then place it on two jack stands.

With no air lines connected and the air tank drained I would check for free turning wheels, both sides.

Then connect the air lines and fully charge the system. Then check for free turning wheels. If the wheels don’t turn freely the relay valve is bad.

To check the trailer service side brakes, have soldier B apply the brakes and see if the trailer brakes come on. When soldier B releases the brakes the trailer brakes should release. If they don’t release or drag the relay valve needs to be checked. It may not be venting.

 

[Duece Man 35a2]

Thanks for the reply, I may have found the problem, but I will keep your info for future reference. I may still have problems and I have more than 1 trailer. Thanks again!!!

 

[Duece Man 35a2]

The verdict is in. The lines where reversed as MWMules suggested they might be. I got a new air relay, but decided to put the old one back in to test it and it leaked air. So I put in the new one and it works great. Problem solved!!! Appreciate the advice from MWMules and Scrounger. Does anyone know where to get the dirty air cleaner element indicator that is under the steering wheel, I can't seem to find one. Regards!