I would like to add some about surge brake maintenance:
As said by somebody else, adjust for brake shoe wear, usually by stars which you can pry with a screwdriver to extend them untill the wheel drags, then slacken a bit.
When thats done, also make sure that the pull rod and bowdens arent too sloppy: When these are too sloppy, you'll ruin the gas strut in the coupling head, causing the brakes to bump.
The biggest cause of poor functioning surge brakes are bad bowdens. as any steel cable, they may get stuck because of dirt, rust or because the steel wires start to brush off. For my 2.5 ton trailer, they cost 13 euro each. replacement should allways be done per axle, and preferrably for the whole trailer to keep the brake force equal to all wheels.
At market sales trailers we renewed brake bowdens every 2 years, because they drove with a continuous 200% overload. (a company had a 5 ton generator kit on a 3.5 ton trailer) At normal use, brake bowdens will last at least 10 years before they age and drag.
when you have one way brakes (backmatic) the backmatic release mechanism likes some copper grease every couple of years to keep it smooth.