Weird braking problem.

This doesn't happen all the time and even off and on with the same trailer but never while bobtailing.

Sometimes when i get on my brakes even with barely enough pedal to raise the needle on my application gauge it feels like I'm going into antilock, with a steady pedal I can feel the brakes hit then release... BUT if I push the pedal down pretty hard I get hard braking.
In the past it had been only with empty trailers but today i was over 80K and it happened several times both using the pedal and the trolley brake at highway speed????

Its also a "come and go" problem it doesn't always do it, but today it happened more than it ever has.

I called my dealership to ask their opinion and was told to check the adjustment, maybe one was trying to camover, but the pads still have plenty of meat on them.

Anyone have an idea? So far im clueless as to where to start looking and at $125 per hr I'm not looking forward to a scavenger hunt.

To me it should be in the antilock system...maybe on the trailer? I do know that all times I didn't hit the pedal hard enough to lock a wheel.

 

Its with multiple trailers.

Today I opened my tractor pigtail plug and the black and white wires had bad connections, I fixed one but the other was missing the setscrew. I pushed that wire in but couldn't find a plug.
I'm hoping the fix is a new plug.

 

That was it... thanks guys.