Brake problem with trailer attached.

rubber or plastic airlines. check the blue air line. you run in the bottom all the time. little bugs can get into air hook ups.

 

Just because you put put replacement parts on the truck does not mean you they were any good, many brake parts are rebuilt and do not work right out of box, that bobtail valve puts alot of air to FT. brakes and alot less to drive axles, when red button is out, you need to look at the crack pressure that should be stamped on the steel tab on top of drive axles brake service valves, if crack press. on valves are not matched you may not be getting any air to drive wheels till you mash the hell out of them, getting a air gauge with fitting on both sides so you can put it on all of lines on side of air brake valves that lead to brake chambers, then ck if air press. is the same as the gage on dash if truck has one if not put one in line and hang it in dash so you can ck. if all lines are getting the same press with red button pushed in?