RD Mack Trailer brake issues

[Midland Oilfield]

I have a 2001 Mack RD688S I am hoping someone can help. The truck brakes are working fine on this truck, but the trailer brakes are locking down after you hit the brake pedal 4 to 5 times. I have hooked a gauge to the blue line and it builds up pressure every time you use the brake pedal instead of releasing it all. I have gone through and replaced the trailer supply button in the cab, the foot pedal, the tractor protection valve (twice), and added a quick release between the tractor protection valve and the glad hands to the trailer. I have also replaced both air valves on the trailer for other reasons, and ran a new line from the tractor protection valve and the quick release to the glad hands just to see if the air line itself was causing it. I have also tested the hand valve and it works fine. I've searched and asked around my area and nobody has any idea what might be causing this, any help from the BMT community would be awesome.

[j hancock]

Does it do the same thing with other trailers or just this one?  That may help isolate whether it is truck problem or a trailer problem.  Bad trailer relay valve?

Bendix has an air brake handbook pdf on the web with all sorts of info

 

[mrgumby]

Follow the air lines on your trolly brake or foot valve there should be a double check valve between the trolly line and foot valve. In the line check that and make sure it good and not leaking internally   Just a thought on something to check

[Midland Oilfield]

There are 2 double check valves on the foot valve, I replaced these valves when I changed the foot valve. Also this is doing the same thing with the second trailer I hooked up to it. I have changed valves on the trucks brake system also just to see if by some chance if air was leaking through. This truck is plumbed a bit different to me. Also I don't think it will matter but this truck started life as a bobtail dump truck and then someone converted the truck to haul a trailer and turned it into a kill truck. We've had this truck for about 10 years and this just started a few weeks ago, so I don't think it has anything to do with the way they rigged it up just giving information. Also I put a gauge on the double check again with this separate trailer and if I have 120 psi in my air tanks the blue line will send around 110 psi to the trailer, and when you let off the brake pedal the valves and the quick release only drop around 30 psi. Then the ABS valve on the trailers will start to slowly open and close dropping around 10 psi every time they open until all the air is gone. I have hooked up to other trucks and trailers with the gauge and when you hit the pedal with them it will pressure up the same but will drop all of the air to zero on the blue line as soon as you let off the brake pedal.

[j hancock]

Never messed with ABS so I am out on that.

Even though the truck brakes have worked correctly in the past, it might be a good idea to get another "built as a tractor" truck and see where your truck is different.  It is clear something isn't releasing but it is hard to say what over the phone.  Maybe a line got hooked incorrectly with all the parts swapping?

[Midland Oilfield]

I know this is very late but the problem ended up being at the tractor protection valve. When we replaced that part we bought one we thought was the same, ended up being slightly different. We plugged off the unused ports on the new valve the same way the old valve was and one of those ports needed to be opened up for it to work properly. Thanks for yalls help.