Something is wrong with my truck brakes when trailer is attached. If I apply brakes hard the trailer brake locks until trailer ABS release. At the time trailer ABS release there is no brake at all or very little on the truck. Trailer swap doesn't helps.
Bobtail brakes is OK. Passed 2 level 1 inspection all good for slack adjusters or shoes. Brake shoes have almost no wear in 3 years. Weird!
Changed treadle valve, dashboard valve, tractor protection valve, nothing helps.
ACom diagnostic all test and calibration done, nothing wrong.
Any ideas where to look?