Regarding semi truck brake itself

Translation needed.

 

Go to the dealer and have them sort it out.

 

Quote:
"truck protection valve"
What's a truck protection valve? I want one.

 

From what you’re describing, your truck is acting like the trailer is forcing the brakes on the moment you hook up. The fact that you can “reset” it by pulling the trailer valve for a few seconds points straight toward a trailer-side air issue, usually a sticky relay valve or an emergency/spring-brake valve that isn’t behaving. Even though you replaced half the tractor’s air system, a bad valve on the trailer can still push air back into the tractor’s service line and make the whole rig stop itself.

Since your speedometer is also reading wrong, don’t ignore the electrical side. Detroit 60 and Columbia models often show bad speed readings when an ABS sensor or ABS module has a fault. That may be a separate problem, but it’s worth scanning the ABS and ECM for codes because they’ll usually point right at the wheel or module causing it.

The quickest way to get an answer is simple. Hook to a trailer you know is good and see if the problem repeats. If the truck behaves normally, the issue is almost certainly on the trailer you’ve been pulling. If it happens with any trailer, then the tractor still has an air or ABS problem hiding upstream.

Start with that swap test and a quick code scan. Once you have those two pieces, the rest of the diagnosis falls into place fast.