My trailer is only 2.1T and I suspect the brakes are less than efficient. When I towed up here with all my machines on it I was right on the legal limit (I had to put some stuff in the back of the 90 to distribute the weight!). I just did not feel it working until I had to brake hard when someone pulled out in front of me. The trailer stayed straight and didn't bother the 90 at all (shook me up though).Maybe this makes more sense.
Maybe I am more aware of it as we have to drive up and down a very steep mountain road, 360 metres difference in altitude in 10 minutes, just to get to the nearest town.
But yes, off the steep roads it doesn't come on unless I have to hammer them. Which I avoid like mad as lock up on a big trailer is not funny at all. At least the gas springs stop this happening. I've only ever had one wheel lock up once on the torher trailer and never on this one, yet!
I think when I get back to driving and towing I will get it serviced and probably find they don't work at all