Electric trailer brakes

check the two brakes that are locking up to make sure the front and rear shoes aren't reversed.

Hi BF,
with the electric trailer brakes I had anything to do with they had a means of adjusting the level of braking to prevent this for different weight or road surface situations--maybe yours is an older controller without this ability.

Knowing the brand of your controller may help to find an operators manual that some one kindly put on line.

Wheel bearing grease saturated linings or rusty drum brake and magnet actuator arm lining surfaces can cause this as I found with one of my tandem Caravan/ trailers when I purchased a used one.
Did the old trick of gently heating the linings several times, with the oxy to make the oil exude out of the lining material until it stayed dry.

Cheers,
Eddie B.

Like, two out of four wheels are locking up? If so, it's like bursitis and Eddie said, it's the brake shoes or linkage, or it could be that the other two wheels are not actuating so you have the controller wound up so much that the first two are locking up

that's what I was wondering about, if the wires were broke to the other two wheels would it cause the 2 to lock up?
The controller does have the gain adjuster and it is at minimum and still locks up.
Need to get it into the shop and pull all 4 wheels to figure out what is going on with it.

are the two that lock up on the same axle? or different axle same side? or different axle different side? i don't think it is your controller..

Having two wheels disconnected won't "make" the other two lock up, because the braking effort is based on the voltage applied to the magnets (which obviously then translates to current) The connected wheels will only use as much current as defined by the voltage so their braking effort won't change unless you have seriously undersized brake and ground/negative wiring on the trailer. Ideal is 10ga for ground (white) and 12ga for brake (blue) for a two axle trailer. I just upgraded my trailer because the previous guy had 14ga for everything.
I would look at the linkage on the brakes and the shoes. I remember Dad's LandRover had worn out brake shoes that would grab occasionally. New set of shoes fixed it right up

Very common to need to adjust controller different for empty trailer and loaded. That is why the adjuster is there. But maintaining the wiring on electric brake is a ongoing thing.

Many trailers the brakes are not auto adjusted for the distance of shoe to drum, so you need to do a old type manual adjustment on the brake shoes. That could be all that is keeping the other brakes from locking as well.