1 wheel locks up when breaking help!

ABS sensor.

 

i would think it might be your relay valve /air valve on that axle--had the same happen to me--everytime i made a brake application over--about 15 lbs the wheel on trailer would lock up--only way to cure it was to release the air from the brakes--push buttons back in and away we go--and just try to brake very gently ---makes for a long trip to boston and back--but if you swapped service lines --should have moved to other side

 

abs sensor generally only tells the ecu what the rate is that the wheel is spinning--and compares it to the others to see if there is wheel spin happening

 

if the brakes are in adjustment,check the s-cam to make sure it hasnt rolled over.

 

Could be a warped drum?

 

The same thing happen to me years back, it was the RELAY VALVE, this was on my trailer

 

and it is embarrasing as hell
heyyyy maybe try and adjust all the brakes--make sure they are all equal
it sounds like an air issue cause as soon as you take your foot off it rolls again--but like the other guy said a brake drum might do that as well