I hope I make some sense here, The length of shaft between the coupler( part that goes on the towball) and the body (the block that is bolted to the trailer)has to longer than the gap between the back of the large nut on the back of the shaft and the actuating lever when the leaver is pulled on hard( the hand brake position)
If this is not set up correctly the coupler will contact the block under braking causing reduced breaking and a thud felt through the tow vehicle as the coupler impacts the block.
My suggestion would be to have as little as 10 mm gap at the back of the nut to the activating leaver when the brake is off and the trailer hooked up to the car.
I found out the hard way that my trailer flexes when stored propped up at the front, stretching my break cables to the point my breaks were running full stroke without any effective breaking.
Take note, I am presuming your over ride breaks work the same as mine.
I hope this is off some help and makes sense.
If in doupt a quick break test on a back road may be a good idea before u hit the way free.