[77newell]

Given the repeated times you've had the brakes examined and been told they are perfect, I would assume that they are just that and the problem is elsewhere. So the problem is either inadequate weight on the axle or the relay/proportioning valve is putting too much pressure to the cans. If playing with the air bag pressure doesn't make a difference then the valve is the problem.

Since the drums and the cans are the same as the drive axle there has to be some down regulation of the pressure to the cans compared to the drive axle. While it would be difficult to do, if you tee'd in a a pair of pressure gauges to the input to the drive and tag axle cans you would know how the pressure compares. On the relay valve on my 93 coach rear axle there is a port that I tapped into for the brakes on the towed that you could use for test purposes. There should be a similar one for the tag axle.