Most of the problems I've helped out with here and personally hands on...were with
the trailer side of the braking system
Poor wiring. From 'just' big enough gauge to El Cheapo connectors. And their
routing to save a few inches to a few feet has some runs doing double duty (gauge
will then be too small for that section/run)
The biggest culprit are the drum/shoe magnet wires. Both just barely large
enough and many times not in good shape...with short potentials
Finally, the shoes need to be adjusted often. The self adjusters don't do the
job well enough
Over on the TV side...the sense point is often not adjusted to have the trailer
lead the TV. GM has a nifty brake pedal switch that snaps over with less than
1/32" movement.
Other OEMs has the plunger type (think door bell button) and has to be manually
adjusted tighter...but the issue is not too tight, as it will false trip