Too much air through service brake line to trailer

No adjustment. Just valves. The red emergency/ supply line should give the same pressure to trailer that truck has on dash gauge up to compressor cutout pressure when button is in. Blue service line is only going to give pressure applied through the foot valve and hand brake valve which is normally no more than 10-30psi range unless peddle is pushed hard to floor. Most hand brake valves are limited to 60psi max but the older Bendix ones had a part number that would apply 120psi fully open. They had tags on them noting the max psi setting.
With spring brakes set ( button out) the red line should be dead from the popped out valve in the dash back thru trailer. If it still has pressure in line you have a valve problem not cutting off air flow.
I would check the tractor protection valve and make sure it is plumbed correctly in and out. If they were straight trucks originally without the red trailer supply valve and the trailer supply option was added later they may not have been connected correctly. Lines my be crossed or in wrong ports.

 

Also. It only takes 10 psi of application pressure to lock up the brakes on a un loaded truck. That is also just enough normal pressure to hold you at a stop light. If you are getting more then that thru the service line to the trailer with normal peddle pressure I would guess the foot valve is for straight truck only and not right for a tractor trailer option. ( different ports with different timed cracking pressures are used).

 

Yes Small/P. May have the best idea on looking at ser. :valve crack pressure, which puts the air to brake at a set pressure that is stamped on the tag on top of ser. brake valve if num. Is low your trailer brake will be getting air way before truck brakes start getting air, you should ck. The crack pressure on the truck service valve to be sure its not way too high compared to trailer crack pressure on the trailers you want the trailer brakes to work a bit before the truck so It stops in a straight line, but the tags on service valve can be hard to read but if looking at a few good ones you can ck them and ck them when a parts man puts one on the counter you will be able to read the crack pressure on tag?

 

A lot of good info there. Some was educational.
Past that I would tee a gauge to the blue glad hand, trailer and tractor drive service hoses at the same time. Some help would be good.
How do they compare with brake application?

 

Loosening the slack adjusters never makes the brakes "softer." The diaphragms in the chambers have a constant area in which the air acts upon. Constant force throughout the entire stroke range. All it does is give the DOT a reason to give you a ticket and makes your unit more susceptible to brake fade.

Were the Freightliners originally straight trucks converted to pull trailers?