For what reason are you putting brakes on all four wheels? You really only have to put brakes on the front set of wheels. I have just put galvanised  calipers on my trailer and made sure that all moving areas were well greased ie. slides, pad guides, etc. S/s or galvanised, maintenance is the key.

You didn't initially say it was over 2 tonne. Still what I originally said, still stands, maintenance is the key. Keep all moving parts well lubricated and wash with fresh water after use.

So it needs to have brakes on all wheels.

Do maintain them well, or so I thought but they keep giving me trouble. Sounds like s/s aren't immune to problems too though.

Boppo, I was not saying you don't maintain them. Unfortunately, salt water is not kind to any metal. All you can do is do your best and hope it stays good. The heat they create also adds to the dilemma, too.

over two ton needs brake son both axles and electric breakaway brakes (in WA at least- unsure of other states)

If you put anti seize compund on the bolts before inserting them this helps a lot when you need to service and adjust the brakes. I retapped all the holes and new stainless bolts and have had no dramas.

but the ones on my 3 ton trailer are bronze with stainless guides and bolts ,no problem with rust on them