trailer brakes won't hold

That's not what I asked

 

At least 105 psi air pressure?

 

How is this possible?
Wouldn’t that mean there would have to be constant air still going to the trailer even after the trailer brakes set??

Maybe disconnect the air lines and see if it still rolls freely

 

The fact that you mention the trailer brakes work properly with another truck tells me the trailer is not the issue, its somewhere in the truck.

Not being able to see the air system schematic specific to your truck makes things a bit more difficult. From what I remember in many cases service air for the trailer is usually supplied from the secondary half of the treadle valve (the valve under the brake pedal). That secondary section is pilot operated by air from the primary section of the same valve and its possible it could be sticking.

My advice would be to get a pressure gauge capable of about 300 PSI and try plumbing it in to that circuit and see if your application pressure is correct. Primary and secondary application pressures should be dang near equal, give or take a couple PSI for the cracking pressure on the secondary side of the valve.

 

Like @AModelCat stated. Either it is a foot valve problem or could be the lines are not plumbed correct into the protection valve causing it to not supply trailer correctly. For whatever reason i’ve seen some trucks that are equipt with an inversion valve or pressure protection valve between the two. Those can be bad.

 

Did he hook another

He needs to hook up to another trailer and see if the other trailer works like it should ,
Or if it’s the same problem .

that would tell
You if the problem is with the tractor or with the trailer .

 

Let's narrow the problem down... When dynamited, does every trailer wheel roll freely, or only one or two of them?