It's really worthwhile installing a brake flushing system on your trailer, so the brake drums can be flushed with fresh water after being dunked in the briny. Kits are readily available but somewhat pricey. The most widely used is supplied by Al-Ko, who are not noted for budget prices. Their single axle kit is over £40, and the tandem one about £50 (more from some retailers).
I bought a tandem kit when I had a 26X, and looking at what you actually get, the price is a bit steep. So when I decided to do the same to my current single axle trailer I looked at just getting the bits. I sourced most of it on eBay, and the total cost is less than £20. I was also able to create a superior configuration to the Al-Ko kit.
The parts needed are two unions to screw into the brake backplates, a T-piece to split the water supply, a union to connect the pipe to a standard garden hose fitting, and sufficient plastic pipe. For a tandem setup you need two extra unions for the backplates and an additional connector to achieve a 4-way split. These unions are all stainless steel with push-fit sockets for the tube, apart from the hose connector which is brass (or could even be plastic).
Some brake backplates already have a ¼ BSP threaded hole for a union, but if not you need to drill and tap the backplate in a suitable location. You'll need a 12mm drill bit and a ¼ BSP tap (this also applies if you buy a flushing kit).
For the backplate unions I've used banjo fittings, which allow the tube to connect at 90°. This avoids the tube fouling the chassis as the suspension moves, which can be a problem with straight connectors. I've also used a 12mm/10mm T-piece, allowing a larger bore tube from the hose union to the T for improved flow.
Regarding the plastic tube, this is available in different wall thicknesses, you only need 1mm. This means that 10mm OD tube is 8mm ID. Thicker walled tube is intended for compressed air, but for our purposes would just reduce the rate of flow. 12mm OD tube is widely used for plumbing in caravans and campers so is readily available from caravan accessory shops, but 10mm is probably easier to get on eBay.
See the photos below. As indicated, click on a picture title to see more comments related to each photo.