Brakes are a real pain on trailers as they are seldom used all the time. Boat trailers are a classic for this.
I use mechanical disc brakes on all the trailers I build, that require brakes.
They have a stainless steel cable that is activated with a galvanized mechanical override hitch to the galvanized discs/hubs on the axles.
The disc rotors/hubs are fully galvanized except for the braking surface and the pad assembly is all hot dipped galvanized with stainless steel bolts, pins and bushes.
They are easily adjusted in seconds with a shifting spanner and run friction free when not activated.
They are reasonably priced for the job they do and are easy to fit and set up from scratch on the axle. They come with galv backing mountings that you weld to the axle to carry them.
Mine were obtained from "Martins trailer parts" in Perth.
http://www.trailerparts.net.au/assets/catalog.pdf Page 35
C2303A CALIPER MECHANICAL - TRIGG or
C2303C CALIPER MECHANICAL - ALKO
Then choose your hubs to suit.
C2106B STANDARD HOLDEN MECHANICAL DISC 260mm
C2107B HQ HOLDEN MECHANICAL DISC 260mm
C2108B FORD MECHANICAL DISC 260mm
C2110B VELOX WHEELS MECHANICAL DISC 260mm
C2112B JAP MECHANICAL DISC 260mm
C2114B L/CRUISER 6 STUD MECHANICAL DISC 280mm
You won't be disapointed with these beauties!!!!