random trailer brakes locking

[Izzys11]

2012 freightliner with a 2000 sun valley trailer. while driving in traffic around 20mph trailer brakes have locked up on 4 different occasions.
i was not touching the brakes when they locked up, air pressure was full on both tanks, no oil or water in tanks .It is an instant lock when it happens but a slight tap of the brake pedal will release them. Four way valve on trailer was replaced along with the emergency valve on trailer. Went a couple thousand miles with no issue then it randomly locked again and instantly released with a tap of the brake pedal. Any ideas ? Maybe coming from a valve on the truck?

[Rawze]

Could be several things .. but this is the first thought that I had when reading your post ...

If the pads are getting thin (or the drums the wrong size), the S-cams can roll-over, get stuck, and cause a lock-up. Tapping on the brakes will often release them again.

[JimT]

Rawze's suggestion should be looked at first. It's both the easiest to check and the most dangerous if left uncorrected. But here are some thoughts, should that not be the cause.

That also sounds like the clueless handiwork of those early automatic emergency braking systems, especially when in traffic. If you have one of those forward radar/collision mitigation systems then it could be the source. Those old ones were very prone to misalignment and malfunction. However, if it was due to misalignment then tractor and trailer brakes should be actuated then should release on their own. Malfunctions should cause codes to pop up, but not always.

And that also ties in with the ABS, as do roll stability systems and traction control systems. Because of these safety features the ABS system can actually activate or release braking on any wheel. At least on the tractor, the trailer might be an all or nothing application as directed by the tractor ABS. Which means there could be an ABS issue, possibly on the trailer, or more likely with the system on the tractor.

Finally, while I'm no expert when it comes to the air system on a truck, i assume it's possible one of the brake valves in the tractor is failing to close completely and leaking air into the trailer service brake line. This might actually make sense if it only happens when driving in traffic where you are lightly on and off the brakes a lot.