Kenworth w900 breaks are not releasing

[arazdolgin]

So what I found. Brakes on bobtail is working good (stopping/driving) with light breaks on huh. When I hooked to the trailer with red air hose, I can drive and breaks on truck/trailer released. When i attach blue air hose, breaks not releasing. I cannot drive with blue air hose. So some problems with blue air lines or some valve attached to it.

Changed brake pedal valve, dash hand break valve. Might be Tractor protection valve? Bought tonight and will try to install tomo morning.

Or might be relay valve? If somebody experienced this, please give food for thoughts how that Kenworth system works. Cheers