Some years back I was given some 6000 LBS axles because they had been removed from somthing. All the electric brake magnets were burned and the entire backer plates were rusted. I was told that the haulers of this size wanted to switch to hydraulic as they were more reliable.
I worked with a gentlemen who told me a story about one day he tried to back his hydraulic brake camper trailer up his drive, it would not back up. He forgot to put in the pin that stops the brake cylinder from pushing on the brakes. So his axle was around the same time era as the posters travel trailer.
Backer plates are just bolt on to the axle flange. So all that is needed for hydraulic brakes should be the backer plates, flex line , and a run of brake line up to a master cylinder.
I have looked at but I am not sure I think there is a electric solenoid that pushes on a master cylinder to cause the brakes to apply this was a higher end quality.
Backer plates, flex lines, wheel cylinders and brake line I have changed all to keep my vehicles running.
Trailer Parts Superstore - Electric, Hydraulic & Air Trailer Brakes
This was the first google that I tripped over for trailer brakes.
Craig Clayton